I have an early version Sapphire, ca. 1983, which uses a servo motor. Later versions, or at least most later versions, use a synchronous motor, as I understand it.
I haven't taken the time to dig out my strobe disc, but based on a quick look my Sapphire completes the first revolution in almost exactly the 3.6 seconds Tony cited (I was going to say a tick under 4 seconds before seeing his post). It appears to be nearly up to speed at that point, although perhaps a fraction of a second away from getting all the way there.
Regards,
-- Al