I had 2 in my last pregnancy. The first I think I lost my mucus plug a bit later and the second I started getting contractions a couple of hours later.
It totally varies! It might not do anything at all so it’s in no way a sure thing. I had one with my first and it did nothing. Baby arrived on their own 11 days later. I had three in my most recent pregnancy. After the first I lost my mucus plug over a few days. The second didn’t seem to do anything and with the third contractions started within 3 hours and baby was born that night.
It isn’t an exact science. I had a sweep a week before I was due and it did nothing. Then had a sweep 2 days before my due date and labour started about 10 minutes later.