If the cut feed rate is 500 - 700 IPM then what would the plunge feed rate be?

I would try not to plunge unless you definitely need to, like making holes. If the plunge is at the beginning of the profile, or pocketing machining operation, make the operation ramp down at the start so the end mill does not receive the stress of plunging. Use a plunge rate that is as fast as your z-axis can move. If this is a lead screw, I would recommend about 100 ipm, 150 ipm max. If you are making holes, then you will want to peck to reduce the stress on the end mill. The feedrate can be fast while pecking. Pecking is the movement of a drill machining operation where the end mill will plunge into the material by a small portion and then raise so the chips can be evacuated, then plunge this way repeatedly until the complete hole is made.