Given that you have recently graduated, probably you are still to a degree in study mode and any math that you learned is still relatively fresh in your mind. Also, in some cases people do take over a year to get to their target scores. So starting now is a good idea.
At the same time, likely you can get to your target score within a period of time shorter than one, or two, years. So maybe your best bet is to shoot for hitting your target within something along the lines of three to six months. If you don't hit it within six months, at that point you could extend your preparation time.
That having been said, familiarity with the test can be a big factor in scoring high. So if you really believe that you would just habitually do some work on the GMAT week after week, month after month, for two years, doing that could be a success formula. Analogous to doing that is doing crossword puzzles every weekend. People who do them every week for years get so good at figuring out the answers.
Still, I think that shooting for three to six months is the way to go. Many people drag out their GMAT plans for years, accomplishing little during much of that time.
