There is a same question here but the answer is too specific and is not helpful for users who would like to know the answer
This is covered in the Area 51 faq
Once you've fulfilled your commitment on a new site by asking and answering a few questions, you'll be able to commit to another site. If you are very active, you may be able to re-use your commitment as soon as the second day of public beta. If you aren't active on the site, you won't be able to re-use the commitment until the end of the public beta (up to 90 days).