Thank you for your question.
Your pitbull should be in heat for about 3 weeks but this can be a shorter or longer (4 wks) time period.
What I am sending you here is an excellent website which explains in detail what you're dealing with.
Please let me know after carefully reviewing the website if you've any questions ? Otherwise,just click "accept" and leave a positive feedback as I will for you also. Sincerely,Don