I have never seen them leave registrations open for less than 24 hours. The maximum tends to be 48. They used to open for a week at a time, but have become more conservative recently. Read through the news section of their landing page and look for when the most recent open periods were and how long they lasted. This will give you a feel for the current frequency and how long you can expect to wait. Lately, it has not been uncommon for them to have closed periods of four to six months, with only a two day opening afterward.
I can tell you that seem more likely to pick weekends, Mondays, and holidays, since these are the days people are most likely to forget to look.
The important thing to remember is this: if you want an account, you must check the site every single day. It took me three or four months to get an account. There were at least three times when I forgot to check one or two days, then came back to find that I had missed a registration period.
There's a piece of software called Tracker Checker that TorrentFreak reviewed a few years ago that says it will email you when a tracker opens registration. Also, there's a site of the same name that seems to do the same. I cannot recommend either of these as I've never tried them, and have no idea whether signing up will result in you getting spam or spy-ware.
Even with one of these tools, I would STILL recommend checking the everyday in case they screw up.