Hi, i have a 1 year non-immB visa, do i get 3 months from the date of exit? Or from the 'exit by...' date? My first exit date is the 16th dec, but if i were to go out a week or a few days before, when will they stamp it from?

Thanks.

I assume that you have a 1 year multiple entry NON-B visa. The purpose of the visa is to allow you to enter Thailand, not grant you permission to stay in Thailand. All entries into Thailand , be it 1 or 50, must be made before the date that the visa expires. When you enter Thailand immigration will give you permission to stay 90 days and stamp your passport with an "exit by date". You must leave Thailand by that date or pay an overstay fine. If you leave Thailand on or before the 16th of December and stay out of the country for a few days, when you re-enter, as long as you visa is still valid, immigration will give you another 90 days from the date that you re-enter. If your visa has expired it will be a visa exempt entry and you will only get 30 days by air or 15 days by land.

That's correct. On every arrival in Thailand with your multiple-entry non-B visa you will receive permission to stay for 90 days. Always check the "UNTIL" date in your arrival stamp to make sure you don't miss the due departure date. Below is an example of an arrival stamp given on arrival with a non-O visa and it is the same with a non-B visa.

It is date of entry, not date of exit, that the new 90 days start from. Entry is day one of the 90 allowed days.

i'm admitted until 15 December, so I can do a visa run on the 15th, or does it have to be the 14th? (The 1 year multple non B I have expires next year). Thanks.

You can depart on the 15th.