As long as you use it for 20 minutes or less. Frequency shouldn't really matter. However, if you pronounced swelling (a penis ring might enhance size a little) or if your penis never feels cold, numb or painful then take it off immediately. A penis ring should increase the size of the erection only slightly.

How often are you using a penis ring?

lately a couple times a week for about 45 minutes

Do you experience any numbness or pain? Is there any swelling during or after use of the penis ring?

no numbness or pain. i don't keep it too tight and it's adjustable

Sounds like you've found something that works for you. Monitor any changes that you see and report to a urologist if you suspect any damage!