If you mean when you can actually schedule an exam, you can do that immediately. I had mine scheduled 10 minutes after I got the e-mail with my ATT number.
If you mean when you you can actually TAKE the exam...well, your ATT should give you a period of validity--a range of dates that you can take the test in. My period of validity started from the day after when the ATT was dated, and lasted 6 months. So technically, I could have taken the test the day after I got the ATT, if there was a testing center that could take me. But I'm not that crazy
You can schedule it for any date within that period...but only you can decide when you are ready (i.e., what date is best for YOU) to take the exam. Also, what date and time you can actually get will depend on what's available at your local testing center. So if you have a particular date in mind, schedule it right away once you get your ATT because it could be filled up fast.