Hi, I’m in Thailand on a 90 day non immigrant retirement visa, leaving in September. 
Is there a time limit before I can apply for another 90 day visa? Can you stay in Thailand on these visas for more than 6 months a year? Thank you in advance. 

If you're only using the 90 day non-imm O visa without utilizing the 12 month extension, then why not just get a 60 day tourist visa with 30 day extension. Same 90 day stay but much cheaper and easier to get

30 minutes ago, Hannon said:

Hi, I’m in Thailand on a 90 day non immigrant retirement visa, leaving in September. 
Is there a time limit before I can apply for another 90 day visa? Can you stay in Thailand on these visas for more than 6 months a year? Thank you in advance. 

As there is no 90 day retirement visa suspect you have a single entry non immigrant O visa (if multi entry type they can be used for almost 15 months with entry/exit every 90 days or less).  As said above it is normal to present financials and extend such an entry using TM7 for one year (and continue doing every year).  No reason to obtain new visa entry very 90 days that way.  But in answer there is no limit to the number or time of use for such visa entry/stay.  But Consulates have their own policy on issue of such visas so that will determine what/if you can continue to obtain.

OP, how long do you wish to stay in Thailand.

There is better options to ongoing single entry Non O retirement visa.

 

BTW: you would need to exit Thailand to obtain another. 

I am leaving Thailand in September, and wish to return again for another 30 days in late November early December. 

2 hours ago, Hannon said:

I am leaving Thailand in September, and wish to return again for another 30 days in late November early December. 

Enter visa exempt and obtain a 30 day extension if required. 

30+30

9 hours ago, Pattaya57 said:

If you're only using the 90 day non-imm O visa without utilizing the 12 month extension, then why not just get a 60 day tourist visa with 30 day extension. Same 90 day stay but much cheaper and easier to get

Tourist visa with extension: minimum 1,000 baht for the visa plus 1,900 baht for the extension.

Single entry Non O minimum 2,000 baht

[If wanting to stay over six months, try for a multiple entry Non O minimum 5,000 baht, but you can use it to stay for over a year.]

Note that the cost of the visa outside Thailand varies by country, but is supposed to be based on the Thai baht figures above, adjusted to local currency at an extremely unfavourable exchange rate.

 

In my view, it makes sense to try for the solution that involves the least amount of hassle. While a Non O will work out cheaper for a stay of over 60 days, the difference in cost is not a significant factor.