When is it optimal to rebuy Kevlar?

At what point should I buy Kevlar in CS:GO? I have a very bad habit of constantly rebuying kevlar every round during a match, and it really drains my funds quickly.

At what point is it safe to rebuy armor?

Armor, as long as you have it, will have the same reduction regardless of how much. Example: 50 armor reduces bullet damage just as much as 100. The only time you need to rebuy is if your armor will drop to 0 during the round (2-3 bullets worth) since 0 armor provides no bullet damage reduction. Rule of thumb typically is around 40, you should rebuy.

Sources:

• http://counterstrike.wikia.com/wiki/Kevlar_Armor/Helmet
• http://www.hattongames.com/2013/03/quick-tips-should-i-always-wear-protection/
• http://www.reddit.com/r/GlobalOffensive/comments/1dzj8h/when_should_i_rebuy_a_kevlarhelmet/