You can decide till 1 min before departure-on the platform or when you see the big screens with all the trains-also with the days you choose to use. IF you take the mobile app, then even the starting day is only decided when you actually start/. Depending on which countries you intend to visit (and be original-avoid the problem-bound ES and FR-too many hassles reported here) you may need to make REServations on top or with surcharge , mostly for the suoerfast hi-speed trains-this you can do also before and without activating the days.

No you can decide even last minute where you want to go or even on the travelday itself Thats the Idea behind these Railpasses

There sometimes the limit of reservations (Mainly France,Italy,Spain) that limit the flexibility a bit (As you need a reservation to go on few trains and few of them have just limited seats for Railpass user)