How long does a Bearded collie cross bull mastiff live?

It depends on the gene pool of both breeds. It is said that dogs with mixed breeds live healthier lives. I've had almost all the dogs in my life that were mixed breeds for that reason (called Heinz 57 variety, but the rage now) and 3 have lived to be 19 - 20 years old. Both of the breeds in your dog need lots of exercise. Be sure you are fussy what foods you buy for them. Gluten content, no filtering organs in their food (could be a diseased animals liver or kidneys in the mix) because as of now there are no strict guidelines for dog food manufacturers. Pet owners are outraged by this last outbreak of bad gluten (from China) and now find that Melamine has been also present in some dog foods. Talk to a good pet store such as Petcetera (if you have one in the U.S.) or any larger pet store.