I still remember a while after NA launch one time I was communicating with a JP-gal-trying-to-learn-English (for some reason, there was a lot of Social shells on Titan that JP people made to invite NA members so they can practice their English, I was in one of them) and she was explaining the concept of 200+ chains.
It went something like:
JP Gal: Highest Chain?
Tanshui: 7, we're so good.
JP Gal: LOL go for 200+
Tanshui: You mean xp? Yeah, we're getting 200+ xp on most kills.
JP Gal: Nono 200+ Chain
Tanshui: LOL, I told you, we're getting 200+ xp.
Yeah, back then never thought getting that high of an xp chain was even possible : O
Nowadays it's like others have already said, 50+ and it's all the same. If I take a party to a camp and we can keep chain going through 3 full repop cycles then I know we can handle it - the chains just start breaking when people lose concentration / real life issues cut in / break time, etc. I have found that when you take any 6 competent people and make a balanced party you can always chain indefinitely.
My static likes to take a 5 minute break every 100-150 kills or so. So ironically, my highest 200-300+ chains came from PUGs.
The only one that I thought was an actual accomplishment was a 200+ chain where a guy hit 10 limit points + cap while he was there (started with a lot I guess) and went back to MH and we continued to kill about 30 mobs and kept chain alive with 5 people, and after he came back another group tried to camp-jack us and we kept chain alive by pulling very aggressively and adding more Wivres to the balance. It was fun.
~Tanshui