Just curious because I have a few people who said they would leave reviews for me last month. Is there a waiting period?

But if it's a first review for either the provider or the client, it might take longer.  You could ask if they really wrote the review.  If they did, they can check to see it's status on their profile.  Unfortunately, you cannot check on review progress yourself.

Oh ok. 2-3 days. Should I re-ask or just leave it be?

Check and see if the client has other reviews.  If he does, and it's been longer than 2-3 days, I'd say go ahead and ask him again.  If he's a good guy, he won't mind, I know I would not mind esp if I had promised to do the review.  Sometimes life does get in the way of things, there's all kinds of reasons why the review has not been posted.  If he has already submitted it, and it's not posted, he can check the status of it on his profile.  Unfortunately, you have no other way of finding out what has happened.

My last one took a month, and I worked on it every day