Never had that experience but it'd be a first time, last dealing with them if it was that shoddy of service. Probably would even consider calling for a full refund, if it was that bad. I understand giving people time to work and even longer if you've known and worked with them in the past, but if they are trying to be responsible, they should let you know if they have a serious backlog/something's come up, etc..

I've had a few situations like this or close to it. It's kind of a personal decision as to when the waiting and poking for updates is no longer worth your time, honestly. I know I'm always still a little salty with a few pieces that I just know I'll never receive - in at least one recent case, the account is completely gone. Othertimes, though the progress is slow - there will still be progress. I've waiting up to 1-1.5 years for a single piece before, to the point that the OC being drawn no longer looked like the sketches and lineart I had received up to that point after design changes. It hasn't happened enough for me to make an actual list, but I do keep a mental one of who works at what pace - and I'm less inclined to work with someone who doesn't have good communication.

Even if they're slow, as long as they communicate, I'm happy. Otherwise, I'm awfully hesitant to commission them again.