What's the lifetime of Wicked Wolf's +1/+1 counters?
The counters will stay; "until end of turn" refers to gaining indestructible.

Note that you cannot "put a +1/+1 counter on Wicked Wolf until end of turn". Putting a counter on a thing is a specific act that happens in one moment; it does not work in English (or in MTG rules) to give it a duration. If a counter were going to be temporary, it would need a separate instruction to remove that counter at the end of the turn, like this:


  Put a +1/+1 counter on Wicked Wolf. Remove it at the end of the turn".