I brought a dress in to be shortened for a wedding 3 weeks away. She said it would be ready in a week and she'd call me. Two weeks later, and no call, I dropped by the store. It still wasn't ready. She said that the fabric had needed her home sewing machine, and it'd be ready in 3 days. So, on my super busy week, I had to take time off work to be able to pick up the dress on a weekday.
The alteration was beautiful, though.
So, if you have a timeline for an alteration, perhaps fudge it a little?