About 25/30 yrs,but they are one of the best of their kind.Just look at all the fault questions on here and see how many are Combi related.The only thing you are likely to need is a t/couple from time to time and what a dream to service.They might not be as efficient as a combi,but you will save a fortune in repair bills.Rip it out at your peril.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix home heating systems.