on average (although i know many breeds differ) how long does it take to house train a dog

It depends on the age of the dog and the amount of time and vigilance the owner can commit to it. It can be done in a week ... but every time the pup is left alone and makes a puddle is a step backwards, so you do need to concentrate 100% on the pup when it's awake. Housework can be done when the pup is napping!
:)

hi the pup is now just over 12 weeks old and we have had him 4 weeks.thanks

Complete house training can take a long time for some pups, others catch on very quickly, and as JG said a lot depends on how much effort the owner goes to to help pup get it right. 12 weeks isn't very old but if your puppy is soiling a lot and you've had him for four weeks it might be worth moving his sleeping area. One pup I bred was wetting every night until this was done. No matter how well the owners had cleaned the old area the smell must have lingered or maybe he had just had got it into his head that this was the right place to go. Once moved he never wet indoors again.