This is a question that I get quite often from new members, and response is practically always the same:
As often as you can!
I write the workouts for our Achieve Group Class as if someone were going to be there 6 days a week.
"What if I'm sore?"
Come in, the absolute best way to get past soreness is to move. The warm up may not feel great, but I guarantee that you'll feel better walking out after the workout than you did walking in!
"But I don't want to overdo it at first."
I completely understand, but this is honestly more of a concern that should be met by how you are recovering between workouts - getting enough rest and nutrients - than it is about limiting your exercise or activity.
Ultimately the more often someone steps foot inside the gym, the greater their chances for success and the less likely they are to be dealing with stiffness and soreness. After all, the rolling stone gathers no moss.