Tyres, hot or cold, pace of rider, temperature of track....that said, 31f 29r probably an ok place to start with most
normally around 29 front, about 4PSI down on road, the rear quite a bit lower than normal, down to 30psi perhaps, mainly because manufacturers set the rear normally at high 30's to account for passengers etc. It's really hard to tell you though as all tyres are different. As a general rule drop them 3-4 PSI and try it, but if you go any more than that it will freak you out a little at first as the tyres will feel a little soft. Some tyres like the Dunlop ones are designed to run as low as 20PSI in the rear, which is why you should set the road pressures as per the tyre recommendation and then just drop them a few