Maximum autonomy for expert glider pilots?

If a glider pilot is perfect in every way, how long can he remain in the air?
It's it reasonable to say that there will always be conditions near his
aircraft that will allow him to gain or maintain altitude, or is it a roll of
the dice?

I ask because I read stories about gliders going on and on for hours and
hundreds of miles or more, which implies that there must always be some way to
gain altitude, provided that a pilot is sufficiently skilled. Or are these
pilots just lucky as well?