How much damage constitutes an assist?

I believe (based on much playing) that you have to deal at least 50% of the total damage the player received. This would jive with what most professional leagues consider an assist. So for instance player A deals 60 damage to player C and player B gets the kill dealing 50 damage. Total damage to Player C = 110. 60/110 = ~.55. Now if player A deals 90 damage to player C and player B deals 400 damage to player C (awp headshot) player A will not get the assist because total damage = 490 and 90/490 ~= .18. This is also why you never see a "double assist"

From what I've seen you need 40+ and you get the assist. My sources - 80 hours of play time.