Mountaineering gear and hiking equipment is not cheap. Fortunately, if you buy good quality gear it will be well made and should last many years. This is especially true of your hiking boots, which will take a pounding on the trail and, if looked after, can give you good service for a long time. If, on the other hand, you neglect your mountaineering boots and allow them to soak through and not dry out, then they will soon be destroyed.
Your expectations of how long your hiking boots will last, and how much care to put into them, will depend largely on what sort they are. If you spend a good deal of money to buy high quality leather boots then you really can expected them to last for a really long time. In turn, however, you do need to take proper care of leather boots. The first thing you have to worry about is drying them if they wet through. The worst thing you can do is put them on a hot surface or in an oven (trust me, I know someone who tried this) to dry them out quickly. All you will do is bake and destroy the leather. Remember that leather is a breathing, living material that needs to be treated with respect and properly cared for. Dry your boots by putting them in a warm (not hot) place with some newspaper stuffed inside. You also need to care for the leather itself. Regular treatment of the boots with an approved sealant or wax is essential. For older boots you want to use a penetrating sealant such as saddle soap in conjunction with wax on the seams. Be careful not to overdo it. I did with one particularly treasured pair of boots and ended up rotting the stitching.
Fabric boots are in some ways more forgiving, but probably won’t last nearly as long. Most fabric boots rely for their water resistance on some sort of lining made of Goretex or eVent. This makes them lighter than leather but also less long-lived as even a small tear in the membrane layer can ruin their waterproofing abilities. Fabric boots should be washed and coated with an approved water repellent. Don’t use anything oily or waxy as that can clog the membrane and ruin its ability to breathe. Also be sure to shake them out as small bits of stone or grit on the inside of the boot can work their way into the waterproof layer and destroy it in a flash. But with good care, your hiking boots should last many years and give you endless enjoyment.