The first thing to remember is that as with traditional IRA withdrawals, if you withdraw from your SEP IRA when you're under age 59½, your withdrawal will be subject to a 10% tax penalty on top of regular income tax (we wouldn't recommend it!). 

If you're 59½ or older, you can start making withdrawals from your SEP IRA without a tax penalty, and if you're 70½ you have to start making withdrawals.

The rules around withdrawing from retirement accounts are a little complicated, so please refer to the IRA website or get in touch by submitting a request if you're thinking about making a withdrawal and aren't sure what the rules are.