Whatever you can handle, it's up to you. Pick something where you are going to do it to failure--in other words, where you can hardly do it. That's the key. So many people will just take five pound weights and do something 10 times. What are they getting out of it? Nothing! Say you are going to build up your bank account. If you put in a penny a day, it's going take a long while. It's the same with exercise--the more you put into it the more you take out.
Why do I talk about the benefits of failure? Simply because failure meant a stripping away of the inessential. I stopped pretending to myself that I was anything other than what I was, and began to direct all my energy into finishing the only work that mattered to me.