Sometimes, if you're really lucky, popsicles will be had. Seriously though, it all comes down to maintenance. Depending on the mods, some will lengthen the bike lifespan, some will shorten the hell out of it. If you are going to get a bike that's been modded to hell and are unsure of it's mechanical integrity, have a mechanic or someone who you implicitly trust that has moto modding experience to look it over. Unless you rip that thing apart before you buy it (good luck trying to do that) there are a lot of things you might miss that the mech would pick up on. As far as maintenance goes, get a list of the maintenance that should have been done and ask which ones have been completed, then attempt to verify either through records or visually. Either they did the maintenance as required, they skimped out (walk away), or they told you they did and BSed you when you see they didn't (run away).