It depends on the material and the quality and accuracy of cut you want.
For prefinished 3/4" cabinet plywood I use a 3/8" mortise compression bit and climb cut almost the full thickness of the plywood at 12000RPM (spindle) feeding at 6 inches per second (IPS) leaving just 1/32" of the plywood not cut. I then conventional cut the last 1/32". This enables me to compensate for bit and machine flex. I use mortise and tenon joinery on my cabinets so this cutting accuracy is critical.
When doing 2.5D carvings, I sometimes use a 1/4" ballnose (other times a 1/2" ballnose - depending on the size of the carving) to rough cut and in soft wood will set pass depth to 1/4" to 5/16" at around 4IPS. In hard wood (bubinga for example) I might set pass depth to around 1/8" to 3/16" at 4IPS. If I'm cutting a part in alder with a 1/4" compression bit, I will set depth of cut to around 1/2". For my finishing pass I use a 1/8" ballnose. It is only removing about 1/16" material so I buggy along at around 5 to 6 IPS.
What it all boils down to is calculating your best router speed and feedrate and matching that to your bit and material. It is a matter of trial and error as each machine will be a little different. If you go to Onsruds website, they post feed/speed calculation sheets for proper chip loads and you can start there and then fine tune for your machine.