The Intel MacBook was introduced in 2006 and SnowLeopard discontinued PowerPC in 2009. So 3 years seems to be about what you can expect before there's no OS, and another 2-3 years after that the patches stop.

I mean, if you had a Motorola Mac (68XXX) at the time of the x86 transition, you must have gotten a good decade of use out of it...

I kid, obviously. I've never seen anybody refer to the PowerPC as Motorola machines even though Motorola was part of the PowerPC alliance.