1. You can contribute up to 25% of your compensation
2. But not more than $51,000 (for 2013; $52,000 for 2014 and subject to annual cost-of-living adjustments for later years)
The problem is that you often don't know what your AGI is until you do your taxes which may be past the funding time period of January 1st to April 15th of the following year. So as a result the IRS allows you to fund the SEP IRA up to whenever you finish your taxes, which the latest is October 15th.
Be advised that you should never over pay your SEP IRA account or else there is a 10% over the amount that you could legally fund.