The short answer: a lifetime - and then some.
But before we get ahead of ourselves, you’ve got to know that how well your leather will handle wear and tear - essentially, how long your leather will last - wholly depends on the quality of leather you start with.
First off, faux leather is not leather. We won’t waste our time or yours trying to guess how long it might (won’t) last.
Bonded leather is the 4th grade of leather quality. Made from scraps, it is quite literally the bottom of the barrel. It is weak and degrades quickly with use.
Genuine leather is in 3rd place when it comes to leather quality and also does not stand up well to stress.
In 2nd place for quality, we have Top Grain leather. Despite its name, this grade of leather is not the top of quality, but it is quite strong and can be very durable. But, it only remains durable as long as its finish remains unbroken. Top Grain leather does not age quite as well as full grain. Basically, it will last you a long time, but it’s not going to look quite as good doing it.
Finally, we have Full Grain leather - the best leather money can buy. It far outlasts its counterparts in long term endurance, and more than that, the more you use it, the better it looks. Given the proper care, the same wear and tear that would wear out and tear up another product will instead develop in leather the character and patina that make it even more valuable and desirable over time.
In our current throw-away, disposable, consumer society, most people can’t imagine a product that lasts a lifetime - and even improves with age. But, at least with quality leather bags, this is definitely the case.