I know people here almost universally reccomend cycling Sulbutiamine, however, I've been looking through the major studies regarding it, and they all seem to be for chronic usage, at least 2 weeks, often longer.
Reading the listed effects on Wikipedia, it sounds like something definitely worth taking regularly:
"Sulbutiamine is a lipophilic molecule that crosses the blood–brain barrier more easily than thiamine. Its metabolism in the brain leads to an increase in the levels of thiamine and thiamine phosphate esters.[1][21] While the exact mechanism of action of sulbutiamine is unknown, it is thought to occur through the upregulation of the reticular activating system, which is the center of arousal and motivation in the brain.[4] The administration of sulbutiamine potentiates cholinergic activity in the hippocampus.[14] It also potentiates glutamatergic activity in the prefrontal cortex through a reduction in the density of kainate glutamate receptors, which may occur in response to a modulation of intrasynaptic glutamate.[22] The facilitation of central glutamatergic transmission is a likely explanation for the ability of sulbutiamine to improve memory.[23][24][25][26] In addition to its action on cholinergic and glutamatergic transmission, the administration of sulbutiamine reduces the release of dopamine in the prefrontal cortex, which increases the density of D1 dopamine receptors through a compensatory mechanism.[22] The modulation of dopaminergic transmission may also contribute to the ability of sulbutiamine to improve memory.[27][28][29] A possible explanation for the pharmacodynamics of sulbutiamine is the increased availability of thiamine triphosphate (ThTP). Although the full physiological role of ThTP is unknown, it is an integral component of synaptosomal membranes,[30] participates in the phosphorylation of proteins,[31] and activates chloride channels that have a large unit conductance.[32] The activation of chloride channels by ThTP may be involved in the modulation of receptor binding."
People seem to report that in the short term, it causes them huge boosts in motivation, but if they use it for more than a day or two in a row, they disappear. This seems to be the basis for the "builds tolerance really fast" theory, however it seems more likely to me that people reporting such effects are actually experiencing the placebo effect. I find that whenever the placebo effect is responsible for amazing effects, it's usually on the first day I take it, since I'm regularly thinking "Am I feeling anything?". Usually by the next day I don't think of it nearly as much. If the effects seem to persist despite the novelty fading, then I assume it's likely producing genuine benefits. However, if I suddenly stop feeling anything, it's likely just the fact that the initial excitement has worn off.
It seems unlikely that such tolerance could build so quickly to a non-stimulant (it's literally just a B vitamin that is capable of directly crossing the BBB), even aggressive medications like dopamine/GABA agonists usually take at least a week to produce any real tolerance/addiction. In fact, the evidence seems to point to it's effects being more subtle, proactive long term effects, such as modulating rather than dramatically increasing neurotransmitter activity. It also seems to upregulate several systems, suggesting that it could in fact have temporary negative affects (as a result of reducing dopamine), but with positive long term effects. Personally, I'm much more interested in upregulating receptors than finding various agonists. It seems like a more sustainable, proactive approach.
So what I'm saying is, that the temporary huge motivation boosts people are actually just the result of the placebo effect (or amplified by it), and that the real benefits arrive after at least 2 weeks usage. One other possible explanation is that people who have a mild B1 deficiency experience a rapid correction of that deficiency, as Sulbutiamine provides B1 directly to the brain bypassing intermediary metabolic processes, whereas traditional supplements require that it be transported indirectly to the brain, and even still according to this passage from Wikipedia's Thiamine article, it's normal form has a low availability to the brain:
"Uptake of thiamine by cells of the blood and other tissues occurs via active transport and passive diffusion.[1] The brain requires a much greater amount of thiamine than in other cells of the body. Much of ingested thiamine never reaches the brain because of passive diffusion and the blood brain barrier. About 80% of intracellular thiamine is phosphorylated and most is bound to proteins. In some tissues, thiamine uptake and secretion appears to be mediated by a soluble thiamine transporter that is dependent on Na+ and a transcellular proton gradient."
Hence when Sulbutiamine is taken, this deficiency is immidietly corrected, allowing previously low neurotransmitters to return to baseline, which to someone who had dealt with neurotransmitter depletion for a long period of time, would feel amazing, almost euphoric at first (sort of like when you manage to clear clogged sinuses; an intense feeling of relief), but you would quickly adjust to the effects. That's not to say they're not still there, you just stop noticing the fact that your mood has stabilized. This also explains why some people react dramatically while others do not; someone who already has sufficient thiamine levels in the brain would probably still notice some benefits (according to the studies), but they would be much more subtle (though still quite interesting, it seems to positively affect several systems).
In my own personal experience taking Sulbutiamine, I noticed only a mild immediate affect, but ever since I started take a normal dose daily (200 mg morning, 200 mg afternoon, 400 mg total), I've been feeling a lot better lately. More stable (I have ADD-Pi, as well as mild comorbid depression at times). This seems to line up with the expected effects from the various literature. Just keep in mind that as it's fat soluble, it should be taken with a fatty food (but low-fiber, as foods with lots of fiber can reduce absorption when taken simultaneously with medication).