I'd guess only a couple amps. Generally lathes designed for school/colleges are pretty detuned to try and minimise damage during the inevitable crashes.

Manuals should be available for the drives (just search for parker CD20/CD40), so you should be able to work out what current they were set at, if there's not a chart on the drives themselves.
Failing that, I'd start about 2 amps, and monitor motor temperature. After 20-30minutes, at peak current the hottest they should be is just verging on not being able to hold your hands on them. If they heat up more than that far sooner, drop the current. If they're still cool to touch, you can increase current if you think you need more performance out them.
If they're old round body steppers, upgrading them to newer square motors will give better performance, but gains are probably not that great on something relatively slow like a lathe. Upgrading the drives is where you'll get the biggest improvement anyway.