How long should I mark my stay duration for a Non-O Visa application while in the UK?

With a Non O Visa you get a 90 day permission of stay in Thailand as stamped into your passport by immigration when you enter Thailand.

You can then by visit a local immigration office to get an 60-day extension based on family basis (Thai wife). I have done so several times myself. You then can stay 90 + 60 days. It is only allowed to do a such 60-day extension one time without leaving Thailand (at least according to the local immigration office I attend).

I recommend that after you have entered Thailand visit the local Immigration office that you intend to go to for either do a 2-Month or 1-Year Extension of stay. Get their list of their specific requirements as it might differ depending on your location in Thailand.

I would guess that you will not stay in a hotel but for example your wife or relatives house in Thailand at your stay.

You need thus within 24 hours after you have arrived to Thailand visit the local immigration office to do a TM30 Address Reporting. Immigration will thus put a receipt into your passport that they will check if and when you come back for do a 60-day or 1-year extension of stay.

The visa gets you stamped in for 90 days, so just put 90 days..

It doesn't matter what you intend to do after you're here extension wise