Quoting I'm Marbles:" I paid for all but one of my bridemaids dresses. I got my wedding dress 3 months before my wedding."
It also depends on what time of year you are getting married. I went for mine in early March and they said that was very late to be buying for needing it by end of May. That was at David's Bridal. It arrived in time but I would suggest going at least a month sooner so you don't stress. Also, if you need any alterations that will require longer, too. My mom did my alterations so I didn't have to worry about a queue for that.
My future SIL was my maid of honor and already had a dress she'd worn for a friend's wedding and was planning to wear the next month for another friend's wedding so that's why I went with that color. I didn't offer to pay for the other ones though and they tried to get ones that they could wear again. I told them to pick anything in that color, so they each had a different style and I hoped it was something they would wear again.