Or just call voice mail and listen to the prompts. That's usually in the "additional items", or whatever the particular carrier calls it.
But ... it's set in seconds, not number of rings, and the number of rings it hears aren't necessarily the number you hear. so decide on the number you want, multiply by 5 (5 seconds from the start of one ring to the start of the next ring) and add 2 (for the difference between what the VM server hears and what you hear). That should get you right on the nose.