Ok, I realise that this is a piece of string question, but I was surprised when all 4 of my car tyres needed replacing last week - the car is just over 2 years old.
I didn't think I was a particularly 'heavy on tyres' driver, and I check my tyre air pressure every 2-3 weeks. The use was general wear and tear, not due to mis-alignment or anything. (One had a nail in it which is pretty much unavoidable - GOK how long it had been there !) I'm a low mileage driver too - no more than 8000k a year (usually about 6-7k)
I was under the impression that it was every 4 years or so, one or two might need changing.