what is the approx raceable lifespan on an alu frame anyways? 3 years? 5 years? more?

looking to the future as one has to when on combined finances, would a better plan of attack be to keep up to date and everythng servicable on a current bike then upgrade the frame when its had it, or just leave the current bike as is then buy a complete new rig when it packes it in?

personal thought puts the road bike up for replacement in 4 or so years, the mtb in about 4 - 5 years as well.