It will depend entirely in the dress and how much you need altered. And the dress maker. My wedding dress took about 1 week - but I only had to have it taken up, straps adjusted and bust cups sewn in by hand. She also only had a couple of other dresses in. Things like rouching, lace/bead work, adjusting around zips etc will all be more time consuming to take in. Taking a dress in will be much easier than trying to find a work around if it is too small. Also, perhaps talking to your intended dressmaker about their opinions regarding sizing and alterations and timing may help. Most people get theirs altered in the weeks before the wedding, especially if they've been losing weight.