Battery capacity. What's nominal?

[ChILd_ReBoRn]

So, since I'm trying to built a rc vehicle on a budget, I decided to build my own battery. Not sure how "big" should it be. I've ordered the PCB, and I think 3000mAh should be enough (3S). I have 2 motors and 2 servos. The dozer is about 30cm big, 6 lbs max. Will it be able to have a 30 min play cycle? Or should I just try to cram in 3S2P battery? That would be massive 6000mAh, more expensive, bulky, PCB won't work for balancing and it would be heavy. Even charging would take a long time..

I'm inclined to 3000mAh, but open to suggestions.

Ty