Army bonuses are a lot like the stock market, they go up and down and you can't really accurately predict when they will go up. I will say it is more likely to go up after the fiscal year (october 1st), but there is no guarantee.
The bonus also depends on what MOS you do, and how long you sign up for. If you are not totally stuck on being in the infantry, ask your recruiter about signing up for a "star MOS." Those are MOS that are short on people, so the bonuses would be higher, the promotions would be quick, and you would be more likely to get a re-enlistment bonus later.
I did have a friend who re-enlisted in September and he got $10,000, but I waited until March and I got $22,500 for signing up for the same amount of time.
