Outside of the summer period, SFMM is open from ten in the morning each Saturday and Sunday. Closing times vary, getting earlier as you get into winter. It can actually be quite chilly there in December (it's way over 100F just now!), so you are generally glad to leave by 6 or 7.
By the way I forgot to mention in an earlier post they have a scheme called the Twicket. When you get in go to the Twicket counter (to your left after the turnstiles, and for something like 3 or 4 dollars they give you another pass valid for another visit inside of a month. So you can get the second day of a weekend for very little (if you feel up to it).
