So many variables; trail conditions, weather, type of lube being used..
When I used wet lube, my drivetrain would get messy pretty fast. My bike probably needed a drivetrain cleaning every 2-6 weeks. I invariably put if off though, because it took a lot of effort to clean up a dirty drivetrain coated with dirty thick oil.
I switched to light lube (Rock N Roll Gold), which I apply before every ride. Rock N Roll Gold doesn't pick up contaminants like wet lube, so now I only degrease the drivetrain 3-4x/year. When I apply the RnRG, I wipe off the excess with a rag, which also cleans the chain. I also spray my bike down after some rides with the hose, and wipe chain and cassette with a rag (no degreaser or brushes though).
xroadcharlie:
If you use a mild degreaser (citrus, etc), and don't direct water pressure at the seals, your bearings will be fine.