If one has an old frying pan which was used for milk/cheese but hasn’t been used for a few months, would it be ok to treat it as a pareve pot? Is it only ok b’dieved? Would it be more permissible if it was more than a year since it was last used?
A milchig pot is still considered milchig even after a few months of disuse. There is an opinion that after a year, that it can be used b’dieved. Meaning that if it was already used, that it won’t prohibit what was cooked inside it. The best thing would be to kasher it if you can. If you can’t, you would be able to heat up the frying pan, and do libun kal on it, since it didn’t absorb anything that wasn’t kosher. This can be done even if it is if the pan wasn’t used within the last 24 hours.
Sharei Teshuva O:CH 451-1 who brings Chacham Tzvi. O:CH 451-4.