Can I pick up my Mercenaries later if I tell them to wait?
Solution 1:
As long as you don't keep your follower waiting for days, your follower will still be there when you return.
From UESP
If your follower is left to wait for a long time, a message will appear that the follower has grown tired of waiting and will start walking to their originating location.
If you have not received that message, then your follower may have wandered off nearby chasing a hostile target, or glitched.