Run time is mostly dependent upon the MAH rating of a pack. The higher the rating the longer the runtime. I can get anywere from 15 to 20 mins of runtime from a 4200 NIMH battery on a MM 5700 system so the same can be assumed for a VXL geard for about 40 MPH top speed. Thats just a round about figure for you to ponder over. High gearing will tend to reduce runtime while increasing speed. So the faster you want to go exspect runtime to drop. Hope that helper some.
When it comes to lipos your runtime will increase over a NIMH of the same MAH rating.