That all depends on the strength of your motors and the mass and friction of your machine. Ideally you want to be able to go as fast as possible without losing any steps. There are some motor tuning programs that will help you get in the ballpark. Also a lot depends on your step kernel rate in Mach. Run the drivertest to see what your system can support. Be aware that you can also have resonance problem at some speeds, there are tricks to fix that, if needed. One thing about steppers is that they have a wicked torque curve vs speed. So you need to test and tune from zero through your max feedrate, then at your rapidrate. If you have a resonance between max feed and rapid, you can live with that, but not if it happens below max feedrate. And of course you must never lose a step, no matter what. A general rule of thumb is to adjust velocity up until the motor stalls, then back down about 10%-20%. Then adjust acceleration as high as possible where they can still start from stationary. This will give the best position tracking. If you plan on using constant velocity, everything will have to be tuned really close, for exact stop you don't need to be as precise.