In answer to your main question, generally people inject 10mg epitalon for around 10 days in a row, although this is higher than the experiments in Russia. The reference is here, PMID: 14523363 but I can't find the dose right now; I believe it was 5mg every other day for 10days? There is some information here as well, from page 73 onwards.
https://khavinson.in...s-genome-ageing
Going back to the original in vitro study, DOI: 10.1023/a:1025493705728, the concentration used was 0.05ug/ml for 4 days and this resulted in comparable telomerase protein in fibroblasts to that found in HELA cancer cells (generally the benchmark for the amount you need to stop telomere shortening; I think it is lower than this). You'd have to work out the clearance rate to see how 0.05ug/ml compared to epitalon injections, but at 10mg per subcutaneous injection you'll be much, much higher than 0.05ug/ml initially. So perhaps the dose should be lower?
Of course the downside of using epitalon is that all the research is Russian. Many individuals (myself included) have increased their telomere lengths using it, however.
TA-65 has more research behind it, both in vitro for activating telomerase (a little) and in vivo for increasing telomere length (a little).
TAM818 is claimed to be a much more powerful telomerase activator than TA-65, but there is no published data on it, only the word of Bill Andrews (who I happen to trust). Again like epitalon, TAM818 has been used to increase many peoples telomere length (see the defytime website).
The price of TA-65 and TAM818 are similar.
Epitalon is cheaper, assuming you cycle it. But there are counter claims it doesn't activate telomerase at all, but acts via melatonin as an anti-oxidant.