Hello, and welcome. I am a licensed attorney and happy to assist.
First congratulations on your upcoming purchase! I assume this is an appraisal required by your lender before the loan is confirmed. The contract would not necessarily state the timeframe for the appraisal and, if not stated, a "reasonable" time is inferred. Basically, it should be done as soon as possible to keep with the timeliness of the contract, as a whole. If the process is delayed, such that the intention of the agreement cannot be fulfilled in a timely manner, the appraisal would be considered untimely and reason for the seller to pull out. To be clear, there is no definite due date, and the matter is open for interpretation. As long as there are no unreasonable delays, you should be fine. If you are uncomfortable with the ambiguity, you could add a date or, if the agreement is already signed, add an addendum to the contract specifying a date certain.
I hope this helps. If you need additional information or clarification, just let me know and I'll be happy to continue providing assistance. If I have addressed your issue and/or pointed you in a positive direction, please let me know that as well, and please remember to leave a positive rating when prompted, as that is the only way attorneys on this site are credited for the information we provide.