Repeated freeze thaw cycles damage peptides, so freeze/thawing it 20 times ( 0.5mg x 20 = 10mg ) does not sound ideal. Keeping it in the fridge for 20 weeks after reconstituting it is not ideal either, it is unlikely but possible to get bacterial or fungal growth over that length of time, especially with 20 needle punctures. It would make sense to mix it with bac water and put 1/4 in each of 4 vials , freeze 3, and use each one for 5 doses (or 5 vials for 4 doses each), and just thaw a new one when you need it. Only one freeze thaw cycle and minimal chance of contamination.