I agree with most things said here. We really try not to assist cows, but if the hooves are out for more than an hour and a half with NO progress, we go ahead and help a little. Rarely do we have to get the chains out. Usually when we help, it's just a little bit of a tug on the feet in the right direction when the cow pushes. Either that or it's got a foot back or something. Of course, if we come home and see feet sticking out for God knows how long, we assist...just to be on the safe side. We rarely have to assist though, and we like it that way.
Heifers- I thought that we would have to help heifers A LOT more when we started to breed a little clubby. I guessed wrong so far. Our heifers calve on their own just as much as our cows do. Last year all of our heifers calved unassisted..only about a half dozen, but still not bad.
Cows- Assist when no progress has been made for 1 1/2 hours, or you see something that isn't right (Breech, one leg out but not the other, that sort of thing). More times than not, our cows have the calves within 45 minutes of having the hooves out, but that might just be us.
Heifers- Assist when no progress has been made for 45 minutes or so after the feet are out. ( I know, pre mature for some, but getting that first calf out is pretty important to me)