How many flights from stock cx2 battery

I guess a few hundred; give or take; depending on how well You take care of them. I don't know; maybe 350 before flight times are so short, it isn't worth it anymore. I haven't flown mine since I've got more powerful off brand ones, cause my heli is so heavy.

The stock ones probably have at least 150 on them as of right now and going strong.

If you maintain your batteries properly indeed several hundreds.
If you just chuck them in and out, maybe 50 or even less!
If you store them fully charged, you might break them in one storage periode.
If you over-drain them (under 3.4v per cell!), you can break them too.
If you drop them while hot, maybe you break them too.
If you puncture them, they're screwed.

And so on and so on .

They last as long as you have patience. If you spend 2 hrs charging and are ok with a 6 min flight, then they are still good. If you want to get a 12 min flight, get some aftermarkets. I know I run the aftermarket ones whenever they are done charging and the stocks if I want a few more minutes at the end.