how long do medical bills wait until sent to collections?

sara
I thought I paid it (so many medical bills from fertility testing). I just received a bill sayings it's 30 days late. I'm calling first thing Monday to pay it. I'm hoping it doesn't go to collections. It has three boxes at the bottoms 30 days late, 60 and 90. So I'm guessing 90 days is the cut off. I can't believe I didn't pay it 😩