I can't answer for sure, but I just got accepted into two schools (U of Miami and DePaul) with only one letter, and it was from 2005. You could probably get the prof to at least write it now and maybe keep it on file for a while? Or you can register with LSDAS now (you're good for a few years that way) and have the letters sent in. If you don't want to use them later, you can always make them inactive. Just remember to do that before you apply, otherwise they'll get sent out when the schools request a report. I guess the main point is, if you plan on asking these profs for LOR's, it's probably better to do it now, or the letters will most likely be pretty generic when you do apply.
This way you also won't go through the trauma I just experienced, finding out that a prof never even wrote the letter, a week AFTER saying it would be sent out in a day or two. "But I look at it every day . . . ." Well, great. But that doesn't mean much to the law schools.