It depends on where one takes the training. Soaring conditions. If
there is plenty of Lift, an instructor can get 1 to 3 hrs with one tow
and demonstrate the basics of flgtht. Otherwise, and in the winter
going up in a light airplane first, showing those, may be more
economical, patterns landings included. Even a Xcountry flight could be
demonstrated in a higher performance sailplane or a selflauncher.

If available, airplanes, gliders and rated instructors a mixed training
maybe more economical. Almost half the required time for a power
license could be done in a glider.

Reading the Regs 61 aeronautical experience for both ratings carefully
you can figure it out yourself. Under Private pilot airplane,
whenever it does not say time in sgl engine airplane, it can be done in
a glider. BTW that goes for the comm too.

All this applies tor the US regulations