Only ever got a month temp import permit once & that was a couple of years ago.
The best you can hope for is 2 weeks.
Overstay fine is 200 baht a day, I think.
I've overstayed on numerous occasions & only paid once - US$15 for a 3 week overstay.
The Friendship bridge is strictest about enforcing the rules, & may collect a negotiable fine when you leave.
The other provincial border crossings don’t seem to worry so much when you are leaving. I just left at Pakxan, was 2 weeks over & nothing was said. I've had the same experience at Houei Xai, been 10 days over & nothing was said as I was leaving. So overall I don’t think it is a serious problem.
Right now the problem is getting in. See Link removed
Have you been doing any riding in the North?
If yes, any chance of some Road & Trip reports to share...