To put a specific time on it, the general rule of thumb is 1/2 the length of the relationship (i.e. takes 6 months to get over a 1 year relationship). I think it's phooey but it's the only one I've heard.
Personally, my last relationship was 6 months and 1 week. She gave me a month's warning that she'd be moving across the country to be with her family and start a new career, so I made it clear that we'd just break up.
Well, she left and I had another girl waiting for me that same day (yeah, she *waited* until I was single. I'm that good.) It's more complicated than I let on so ask for details before you judge..
In any case, it all depends on how prepared you are for a new relationship. my first breakup left me emotionally unstable for over a year (3 year relationship). My last one, well, I was over it before it ended.