Hi Kat,
This sounds like a great suggestion / idea to submit to Microsoft. You can get to that microsite by clicking the Ideas link on the menu on this site (and then navigate to the Dynamics 365 Business Central ideas section).
If you are a developer, or work with one, you can also easily create an extension to add a field to the Sales & Receivables Setup where the user can define the number of date that the quote will remain valid for, and then use that value/formula to populate on the quote based on the Document Date.
I hope this helps. If my response has answered your question, please verify by clicking Yes next to "Did this answer your question?"
