Generally speaking, I don't believe that it is necessary to spend an entire day helping out at the glider field; but anyone who arrives after the sailplanes have already been assembled and moved to the launch point should expect to remain until the end of the day, so that he or she can put everything away.
Previous experience on a normal powered aircraft, as opposed to a Motorglider, is a mixed blessing.
It depends very much upon the individual. In my own club, we have had difficulty converting relatively low time (< 1,000 hrs) PPLs and CPLs, who often have bad habits such as fixating on the instruments, and are sometimes incredibly sensitive to negative G (see further Derek Piggott's
Gliding Safety). On the other hand, we have had high time professional pilots (retired airline captains, production test pilots) who have proven to be 'natural' flyers, requiring very little dual instruction.
If you have done no flying at all, learn to glide first. You will be a better pilot for it.
I agree with this, although I'm biased. One benefit about this route is that it tends to make one naturally weather-sensitive. Today I fly power rather more often than I do sailplanes, but I always look at conditions from a glider pilot's perspective; I am never tempted to go 'scud-running', because it would seem so unnatural. As so many GA accidents relate to poor weather, I believe that my background provides an unconscious but important safety feature in my flying.