It depends also on your luck,
gen1 could have a potential between 80 and 120, because the genesis virus has a potential of 100
gen 5 or 6 can have a potential over 200 but I saw a virus at this gen with only 50.
Mutate only the best viruses to achieve best results. So you could end up with something like:
1 gen1 -> 2 gen2 -> 1 gen3 -> 3 gen4
And if I withdraw ETH will it destroy the Virus in the change? Or can I withdraw multiple times from same virus?
You can withdraw as often as you like, but be aware that you have to wait 24 hours after your withdraw from that specific virus again.