Unlimited by air. But if you're staying maximum time each time you enter, be prepared to be asked about why you are staying for so long after your 3 or 4th visit in one year.
If in chiang rai just take a short drive to mai sai. Stamp out of Thailand, buy Myanmar pass for 10 USD, do some shopping (alcohol / tobacco is cheap. If you are a foreigner customs won't bother you generally), come back on a visa exempt