I've been to dozens of barns and seen those where the horse is prepared for the rider, so all they do is walk in and hop on. Then there are barns where the horse, regardless of if its yours or not must not have a speck of dirt or dust on it.
Personally, I spend between 30min to an hour grooming both before and after riding. Not only do I bond with the horse but it gives me a chance to really feel the horse and find any bumps, cuts, sores, etc. It's also quite relaxing for both horse and rider.
How much time do you spend grooming?