Does it take 18 months for twins to be born or nine?

Nine, reason being. A human life take nine month to develop.  The womb of a women can develop more than one life at a time.  

p.s.- it would also take nine months for triplets. 

Nine, give or take. We say nine months, but really, some children are born early, or later than planned. But normally, It takes the same amount of time to create twins, than it does to make just one child. They are both conceived at the same time, so they are growing at the same times, together. If it were to take 18 months, it would be because one twin was created, then the other started creating, by that inference.
Hope this helps!