This varies a LOT by store and manufacturer. David's and Azazie tend to have fairly short production times (0-8 weeks for DB, 6-10 weeks for Azazie?), but other manufacturers can be more in the 4-8 month range. Daughter's Rebecca Ingram dress (a Maggie Sottero brand) from an independent bridal shop was estimated at five months and arrived in four. I've seen posts on this forum from brides where the range was as much as 8+ months. So, you need to think about the kind of dress you're looking for to make a guess on manufacturing time.
I ordered my dress in March of 2019 (which is early) and it arrived in at end of July 2019 and I picked it up in August of 2019. It is now in storage at my parent's house. It really depends on the shop and where you are ordering from though. My place told me it could take up to 10 months to come in because they base it off the wedding date.
Definitely depends on where you order it from. It also depends on the kind of dress too - my Oleg Cassini from David's Bridal took a few weeks to get in, but some of their readymade stock is already in and ready to go. I ordered mine at the beginning of October and had it with me two weeks later - it's currently taking up residence in my closet! Also depends on the color, since some are more of a special order. I would say start looking at least 9 months out, so you have plenty of time to find one and order it. And don't forget that alterations also take time.
Ordered my dress in Sept. (which was about 6 mo away from wedding date) and they recommended Rush Shipping. (I accepted, which added about $200) I am now expecting my dress any day now! (10 weeks was how long the rush shipping would take.) Hope that helps