GWizard is your new friend:

http://www.cnccookbook.com/CCGWizard.html

It depends on the cutter diameter as clearly smaller cutters will deflect/snap if you use too high a chipload. As a rough guide:

MDF:
12mm cutter 0.4mm chip
6mm cutter 0.3mm chip
3mm cutter 0.25mm chip (maybe less)

Plywood:
12mm cutter 0.42mm chip
6mm cutter 0.35mm chip
3mm cutter 0.3mm chip (maybe less)

(Can give more examples if required)

In reality you may end up using a fair bit less than these values as the machine may not be able to achieve these feedrates.