Wow! A truly open question, I'm afraid.
I've heard of bikes that have sat for years, and fired up with virtually no hesitation at all, and seen some incredible things in Asia, where working in the street, or on dust, dirt and sand, by the side of the road is commonplace, but we've also had bikes in that have been claimed to have been looked after (or even rebuilt!) that have been a disaster waiting to happen... and in one case, had already happened!
My personal point of view, and not from a business aspect, but from a practical one, is that the motors, and [new] parts for them, are getting increasing harder, and more expensive to source. I would recommend looking inside it much sooner, rather than later. It will cost less, and be easier; and could possibly cost substantially less, if the centre crank seal looks anything like either of the last two we've had in:
Worryingly, both customers claimed they had come out of [poorly] running bikes! One had been sold his bike, with the seller claiming "it just needs a dyno setup"!
Unfortunately, only one of those was caught in time. The other also ruined the crankcases.
I guess it comes down to how lucky or brave you're feeling, and particularly how the bike was stored. However, we are now seeing bikes that are being regularly used, with seals failing sooner and sooner, which we're thinking could be down to the higher and higher ethanol content in modern fuels.
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Andy.
NSR-WORLD.COM
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