Depends from the wheel, the tire, the use case and the time of the year.
Most Brompton tires are rated to around 100PSI, some higher, few lower.
With the Schwalbe Marathon Racer (Brompton Standard tire from factory) I typically do 100PSI on the rear wheel and 90 PSI on the front.
The higher you go the easier your Brompton will roll and the less energy you'll need for a certain speed. So high is good. Unfortunately the higher you go the more uncomfortable your ride will become as well - so it makes sense to have a little less pressure on the front as this influences comfort a lot and the Brompton is very backloaded, so less pressure in the front has less negative effect than less pressure in the rear wheel.
Checking and adjusting the pressure about weekly is a good idea as the tires will loose pressure relatively quickly.