I don't think there is a percentage like other things. Like cars, watches (time pieces) are a hobby, not just a fashion accessory.
I am a collector and I'm slowly working my way to a more expensive watch all the time, it all depends on how much I save PARTICULARLY for that watch and what my wife allows me to spend. ( fuck you guys
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I have a collection that ranges from a $300 (cdn) Seiko, to a $2k Tag Heuer and what I buy has no bearing on what I make, only what I have to spend on something that in most people's opinion, is useless.