It really all depends. As I can empathise with you on this, I always provide support even after the 6 month period has expired, especially if a happy customer before finds bug now, oh boy! As fast as lightning.
However, with that being said, setting a limit would be bad for both sides.
I sometimes get customers that have a hard time with the English language, and that’s okay, eventually we do get on the same page and I’m able to assist them. Limiting the number of tickets would limit the number of interactions I could have with such customers, and this could easily be abused by authors who say “oh, you already asked xx questions, sorry”. It can have negative impacts on both sides. Again, a win-win / lose - lose scenario. As imperfect as the current one.
On the other hand, I’ve dealt with customers that sent over 500 messages ( and I’m not even kidding ) claiming they have purchased the support extension with no intent of being rude at all, it can really drive an author crazy.
This is a knife with 2 edges, it cuts both ways. Usually major authors do provide support far beyond the line of duty and allotted time, but there are some factors to take into consideration. Some authors have been abused and no longer accept any deviation from the support terms, some authors only provide free lifetime support for small issues, and issues that take much longer they ask for extensions and some authors gladly answer questions long after support has expired. I
It’s not a perfect system. This is very subjective by nature, the terms exist to protect both worlds, yet there’s no 100% objective and perfect way to put support into proper terms to cover the needs of everyone.
However, in your case, you’re best friend is Envato Market Help and Support. If you state there is a bug and the author holds a ransom for a fix, that’s absolutely unprofessional and will be frowned upon heavily. I suggest dropping them a ticket to see what the best course of action for you is in this case.
PS: No offence intended. I can see these things from all sides, being the oldest Moderator here and I can say I’ve seen them all. It’s really a tricky situation, and there is no perfect “fit all” solution for this.
Cheers and all the best!