My Non-Imm B visa expired last month, and I haven't had a chance to get back to my home country to get it renewed. I make frequent trips to SIN and HKG, and I'm now on my 3rd entry into Thailand on a 30-day tourist visa-on-arrival. I've got another trip scheduled in 10 days. Will it be problem to enter into Thailand by air a 4th time (or 10th, 15th, 20th, etc.) with a visa-on-arrival? I've also read about the 90-day limit for tourist visas -- how does this 90 days work? Do they keep a tally of how many days you stay in-country on a tourist visa, or does it get "reset" when you arrive by air?

I suspect you are getting 30 day visa exempt stamps rather than visas but in any case there is no 90 day limit on either. There used to be a restriction of 90 days in a six month period for visa exempt but that has not been enforced since they changed to 15 days for land crossings a year ago.

You're right -- I didn't fill out any application, just walked straight to immigration and got stamped in for 30 days. I'll probably have about 15 more entries before I can get my Non-Imm B renewed, so it's good to know that I won't be denied entry and forced to buy a flight back home on the spot!

I wouldn't put all your hopes up on getting another 15 VOA's the rules could change at any time and then you'll be stuck. Try to get back on another Non-B or a Non-o for visiting friends as soon as possible.