My bitch had a litter last December and the "water bag" was hanging out for 2.5hrs, after which time I took her to the vet and there we stayed until all babies were born.
I've been asking various people about how long the "water bag" should/could/would remain at the entrance to the vagina before the puppies arrive but nobody could give me a definitive answer as it seems that all bitches are different! 
That makes sense but what I really should ask, I suppose, what is the maximum time I should wait  before I start to worry?  Someone I know left it over three hours and the delivery went well but most others I asked said it all came out fairly quickly.

Would appreciate and information although my bitch isn't pregnant at the moment.
Many thanks in advance. 

My yardstick is not to leave a bitch struggling (pushing hard) for much over 1 hour without seeing a puppy delivered, each delivery.  I've never had what you describe re the water bag so can't help you re timing with that other than to say 'fairly quickly' also - sorry.

OK thanks for replying anyway.  She wasn't pushing though - in fact she was almost oblivious to it being there and I had to make her go back to her whelping bed LOL.  It was her first litter though so maybe that's why....just really wanted to know to add to my experience, not that I plan to have much more than one more litter. 
Thanks again

Not only does this vary from bitch to bitch, but also in different breeds.  I have a very tiny toy breed and expect a first pup to be delivered within half hour of seeing the water sac start to protrude.

I've only had 4 litters and have never seen a water bag, it's always burst inside the bitch, but I wouldn't be happy leaving it much longer than an hour after the "gush".

Book of the bitch suggests two hours.