We recommend that you start with a velocity of one-third story point per programmer per week. That is, we assume that every week of time you estimate for development will take three weeks of real time.
This number is a rule of thumb. An experienced XP team will have a measured, more accurate number, and you'll be experienced real soon.
Here are some of the things that make up the number. There are probably more.
You probably don't have all the stories, and some of the ones you do have will change as customers learn more about what they really need.
There will be meetings, reports, writing, support, testing, planning, and various other activities that mean no one will really be able to program eight hours a day, day ...