You must make a concentration check to use the ring.

For regular demons, it will be a DC of 15. For high level demons like Crowley and even Hectate (though not technically a demon), the DC to affect them would be 20. Successful will saves on the part of the demons will negate the attempt and they will be exempt from another attempt for 3 rounds, though you can try with another demon every round. The DC for low level demons to fight off your mind control will be 20. For high level demons, the DC is 15.

If you are able to make a concentration check of 20 or more, you can actually target 2 regular demons at once. High level, of course, is still only one at a time. If you ever get a natural 19-20 on your roll, they do not get to roll a will save unless it's Belial himself, and even then he will have major disadvantages.

A successful controlling attempt means the following:

Regular demons: You can make them turn against other low level demons or other such low level enemies and they will fight for you. However, they will not do self harm and they will not attack their masters. They will, however, run from them if you direct them to. Low level demon control will last the entire rest of the day. You only need to get them once and they're yours.

High level demons: If you are successful, they will be under your command for 5 rounds. They will attack lower and equal level demons for you, but will not harm themselves. So you can pit them against each other nicely. If you (Penny) attack them, then your control will be broken. They will see you as an enemy again.

Belial: Your concentration DC is 22 with him, but his will save is also 22, to negate it. If you get him under your control, you have him for 3 rounds, same rules of no self harm and no attacking him yourself.

So to recap:
Regular demons: concentration check of 15. If you get 20 or higher, you can target 2 demons at once. They must get will save dc 20. Once you have them, they're yours.

High level demons: DC 20. Nat 19-20 roll means they get no will save. Their will save must be 20 to shake off your attempt. You get them for 5 rounds.

Belial: Concentration DC 22. His will save is also 22. If you get a nat 19-20 he still gets a will save, but the DC is 28. You get him for 3 rounds.