Hi B2, 10 minutes is more than enough in my opinion. I think 20 minutes might be too long. Others may differ in their opinions and I would be interested to hear their recommendations.
Typically, if I am just taking a single salt solution enema, I'll usually retain it for 3-5 minutes before evacuating. As you become more comfortable and more confident with the procedure, you will learn what is best for your body.
Listen to your body. If something doesn't feel right, it probably isn't. Start off slowly and make small incremental steps. In no time you will be in a position where you will be able to offer advice to others starting out.