Your best bet is to find the seamstress you want to do your alterations and discuss it with them. From past experiences and all the forums, I called about 6 months out. My seamstress said 2 weeks before my wedding date (which only screwed me up because of the pandemic but otherwise would have been fine)
It really depends on your seamstress. Mine could have gotten everything done in 2 weeks if I wanted (no rush fee or anything, that's just how she works). My dress had to be taken in 2 sizes, straps added, and bustled. Removing the pandemic from the equation, it took about a month because I wasn't in a rush.