im cooking lobster and i would like to know how long it takes to boil it??

Fifteen minutes per pound is too much. It doesn't make sense if you scale up or in multiples of lobsters to water ratio. A quick google search led me to this insighteful Maine website : http://www.newenglandprovisions.com/mainelobsterrecipes/boiledmainelobster.html

Instead of thinking about how many minutes per pound, you may also want to consider how you will eat it when it emerges from the pot. To this question, I submit a chef I worked for many years ago, whose method works wonders for lobster ~ http://www.nytimes.com/2002/04/10/dining/a-chef-invents-a-lobster-dish-and-pots-start-boiling-all-over.html

I am a big fan of the latter method because, although it may seem complicated, you get one hell of a sweet, tender, succulent lobster! And isn't that's what the $ is spent on them for? I like to say a little something (word of thanks/prayer/what have you} before placing my live animals into their boiling death, but that bears no bearing on the flavor.

15 minutes a pound from the point of boiling. Enjoy!