Foil kites last a very long time. They are incredibly durable. That said not all foil kites are equal. Heavier fabric lasts a lot longer than lighter fabric. And newer generation Flysurfers are all lighter fabric. Most people would opt for lighter fabric for the performance benefits anyway. That said the life difference between light vs heavy fabric I would guess is 1/2 to 1/3. The big factors in life span is how and how much you use the kites. If you are careful and gentle with the kites you could get 3-6years out of a light fabric one, or with a heavy fabric foil with insane use and abuse it could last only 3 years. I have seen ancient foils in like new condition! it is all how much they are used.
The biggest issue to an aging foil is porosity or how leaky the fabric gets. The air tightness of the kite overall is critical to how it keeps it's shape. There is a solution to a leaky foil, but it is far from easy, but it does it exist. You basically need to paint the whole kite with a special solution. Which will add a little weight but restore the kite to like new.
There are a few things to help preserve a foil. Not leaving it out in the sun unnecessarily, ie don't dry it in the sun. I would avoid washing or rinsing the kite. Other things to do do are landing/packing on grass, launching and landing downwind,not dragging it or it's lines, folding before you roll the kite and obviously all the general things like avoiding smashing it into the water/ground etc. Your enemies are sun light, abrasive surfaces and stress on the fabric. It's best to not let waves load a kite with you attached. Foils are much more durable in waves than LEI but eventually they do rip, full release and let it go
Lastly if you are concerned with the price of foils, consider instead trying a Pansh, they are the cheapest kites you can get and .. foils