Am sure it depends on a lot of factors...genetics, care, health, etc. Don't have any info on bull longivity, per se. However, Longhorns in general can live into their mid to upper 20's (think some have even lived into their 30's). Have heard that cows can calve into their early 20's without problems.
If you have a quality bull, would suggest that you collect semen on him for freezing...then, if something happened to him at least you could A.I. the females.
Perhaps some other Longhorn breeders on this forum can provide you with a better answer....
You can figure a bull will last till her is six years of age or he will last until he dies if you want to keep him that long. Freeze some semen if you like him that much so you can be certain to be able to breed with him even after he himself is done. Jake