I understand you can roll to recover strain after combat but how long does a character have to wait to roll to recover more? Or do they have to sleep.

Being able to check to recover after combat is just one way of doing it.

Recovering strain is a very flexible process. If you get a number of advantages on a check, you can use some of them to recover strain. Thematically, it represents you feeling successful and confident in the face of danger and therefore more at ease mentally. This can be done on numerous checks. Succeeding an opposed check with advantages is a great thematic way to recover strain. Your PC thinks "Whew...he believed me. I'm not too bad at this deception thing!"

Thanks! I just wanted to be sure i wasnt missing anything.