How long are surgical dentures stitches allowed to be left in the mouth?

Sutures are made of different materials.  Silk sutures can act as a wick for bacteria and should not stay for a long time, but they are strong.  PTFE sutures have less issues, but can break more easily.  There is not much risk leaving them in for extended periods of time, so if your surgeon wants another week, I wouldn't worry.

 There are many types of dental sutures available. Some will dissolve in as little as 7-10 days, while others may take weeks or even months to completely dissolve. The dentist who placed the sutures will be best equipped to answer that question properly.I hope you found this information to be helpful,Dr. Champagne 

Hi FreeSpiritedBoi!
For many procedures we only need the sutures in for a week or two. For some procedures however, such as bone or gum grafts, we may prefer to have them in for up to 3 or even 4 weeks. But not knowing your particular case circumstances, it's not possible to give accurate advice. Sometimes sutures that are self-dissolving can take longer than expected to dissolve (even more than 4 weeks depending on the material and the mouth), and this is not an issue - but if it's irritating the mouth, or accumulating plaque, the dentist or surgeon can remove them before they dissolve.

Non-resorbable sutures shouldn’t stay in the mouth that long. I usually use self-resorbed dissolving sutures for denture patients. Those non-resorbable sutures can harvest bacteria and they are not recommended for months. Hopefully he’ll cut them out for you next time you see him. He may want to keep them in longer because you have a large gap in your wound. If that’s not the case then he need to cut and remove them.