Last year I was sold a pair of Riedell 1310 LS boots, and was more or less assured by the skate shop that they had sufficient support to see me through FS5.
I think really what this means is that the boots, when new, were suitable for up to FS5, not that they would necessarily last you until you were at that level - skaters progress at different paces and since no one has a crystal ball, it's really impossible for a fitter to say specifically "these boots will last until _____" - all they can really do is give you a general idea of what level a boot is suitable for, but even that can vary from skater to skater.
I do think the lighter weight boots break down faster - but that said, how long boots last really depends on the skater and how hard they are on them. I've broken down boots in months that "should" have lasted me much longer. I need a stiffer boot than my skating level would suggest - but I know that now, so I can compensate. I didn't know that as a beginner though... it took me several pairs of skates (and it was only at the recommendation of my coach, not the fitter, who got me into the right level boots).
Being overbooted as a beginner is not good, but if you find yourself able to do everything you want to do in the 900's, they might be just fine for you now - many adults need stiffer boots than younger skaters.