How important is it to run in a brand new dirt bike?

very important... no matter what, your engine parts start wearing down the very first time they move against each other.... each run in program is specially designed to that specific bike to produce the best overall effects (i.e. they'll tell you to run only mid-range rpms durring however long, so that no abnormally deep gouges form, instead you get uniform wear, and a longer life expectancy)

in the begining they lose alot more shavings then after they've been worn in... this is why it's so important to break it in and not over-rev it in the begining, but it's just fine afterwards... the parts will always continue to wear down, but it's early in their lives where they shed pretty much all of their shavings.

well aparently my 125 thumpsta blew up after 5 months becuase i over revved quite a bit when i got it ( so the shop said)

When you run in a dirt bike the top end (piston area) only really requires up to 15hrs

Most dirtbikes have a 30hr plus running in time but this is mainly to run in the gearbox becuase thats in for the looong run unlike the top end

for most chinese dirtbikes a 3-5 tanks of petrol is required before overrevving it

gd luck