i got the urge for more power after owning my club spec evo 2 for just over a year, bought the STi RA which i have had for bit over 2 years now, bought a daily about 10 months ago so i guess i have had something new to play with almost every year since i got my license.
I think there is definately a point you reach when you get bored and want something new, I'd use that as an excuse to modify
But i could see myself becoming bored of a stock car very quickly and going through a 2 year cycle like Kym said.
Hell, if my car was running I'd be very close to seeing if there was any interest and flogging it to buy 'Boosted's 05 STi which is for sale currently...i think i have fallen in love! But then there is that Version 6 STi RA for sale as well...
Must be that time in the 2 year cycle, there are so many cars for sale on pdiddy atm!