Should I wait a set amount of time before going in no matter what
No, we never use a set time to return in, if LO needs you then you go right away.
If you think you are jumping in too quickly and disturbing a mantra cry for self soothing then yes wait outside the door and be ready to go in if it turns to an I need you call or cry. Note, LOs who are responded to don't always do a full on cry, they may call out instead because they are calling for attention, whether it is a cry or a call you return. Mantra cry no need to return but I knew with my DS if he mantraed for more than a few mins he was going to need me so actually I didn't leave him to mantra for 20 mins, 10 max.
Once in, we stay until she sleeps.
You don't always need to stay until LO is asleep. If you feel this is needed then fine, but sometimes you can go back in calm her and then leave again. over the years there have been times my DS needed me to stay until sleep so I have but more often if he called he needed a bit of reassurance (for varying lengths of time) and I'd still leave before sleep.
Sounds like things are going well if there is just the mantra cry.
hope that helps.