Reparing/Replacing Rotten Wooden base for Greenhouse

I think it depends on what the spare timber is, if its something not very durable, in that kind of position you could be looking a 2 yrs, if its hardwood, not all hardwoods are durable, maybe five or six yrs.
Painted on preservative helps but needs to be reapplied to be really effective.

 

I'm lucky with the wooden base of my GH, it's teak that was once part of colliery headgear. It's probably now around 100 years old and there's no sign of rot. The same though can't be said for some parts of the wooden framework, it's about 25 years since I built it and has started to rot in places. I've just spent a few days with rot cure/hardener, filler and then repainting. Fingers crossed it will last for some years yet.