1 full, solid hour is the minimum really for a restful, restorative nap. A full sleep cycle takes about 40 mins, so 1 hour is a cycle and a half.
If you can get a longer nap, go for it (the more rest the better), but if she's happy and doesn't look particularly tired and fussy during the time she's up, I'd say 1 hour is fine.
*HUGS*
Sonya =P