How often do you change tyre of your indoor trainer?

i picked up a Vittoria Zaffiro trainer tire (it’s red) because it was cheap; it stays on its own wheel (swap to one with an outdoor road tire if I use the bike outside). I keep a new tire in a drawer somewhere and change the tire if it blows. I ride 3-4 times per week and am only on the second tire in 5 years. Main wear/damage is lengthwise splitting/cracking, maybe due to heat.