Is there a defined amount of time that the review will take once it have officially started?
I'm assuming that the start time you mean is from the time when you hit submit. That being the time, it will range from 3 weeks to a month to get your review comments typically. It may be less if you are doing a separate submission, it could take longer if you receive an MRC from the review team, and it may take longer due to the time of year you submit it.