I skied a pair of Nordica Grand Prix (the old black racers-only version in a "medium" flex) over 750 days before a huge crack in the lower shell forced me to retire them. I treated them well (Cat-Tracks on the soles when not clicked into bindings, buckled during storage, stored in a low-ozone room during the off-season), and kept the liners (ConformAble foam liners) in good shape. I even took them out of the shells fairly often to prevent water from pooling between the liner and the shell.
I demoed the newer Nordica Grand Prix in 2000, and they didn't fit as well for me, so I looked elsewhere. I have a funky foot that was beaten to death as a junior racer (AA-width forefoot, C-width heel), so finding a proper shell fit helps a ton in making me a happy skier.
Now I'm in Lange L10 Race boots from 2000, and they have about 100 days on them. I need new liners for this year, but my footbeds are still good, and I'm treating them with kid gloves. Aside from some scuffs that have removed the paint-on graphics, the shells are in superb shape. I expect that these will last about 500-700 days, so long as I keep up the preventive maintenance.
But I do cycle the buckles open and shut a lot, as my boots are extremely snug and my foot circulation is not great (due to the aforementioned junior racing days and some wicked frostbite suffered at Jackson Hole and Snowbasin as a teenager that caused some nerve damage in my toes). Thus, they're often open during lift rides. I do have ThermIC boot heaters, which help on the coldest New England days (they were bought for the 2000 Olympics, where we were on the hill before sunrise to get the speed courses in shape).
I'm rambling now. But the key is to keep things in shape: don't use solvents to clean your boot shells; let them dry as completely as possible between ski outings; if you ski in treated snow (e.g. salted or chemically-hardened snow, or dirty late-season snow), rinse and dry your shells and any hinged interfaces before storage; use Cat-Tracks or another sole protector to prevent the boot-binding interface from getting corrupted; store them buckled; don't store them anywhere close to an ozone producer (e.g. furnace, water heater); don't store them in direct sunlight. These steps will help prolong the life of your boots.