How far along before having to wear a belly band/ maternity clothes?

I wore a belly band for the first time today (what a mess) and I'm 18 weeks along. My "teacher work pants" with 3 clasps, a button, and a zipper was just not going to cut it with my recent belly growth today, so I decided to try the belly band... One: it fit on the bottom of my belly, but not the top, Two: because of said fitting, it kept falling down all day and made my shirt all crinkly, Three: bathroom time was just as much of a struggle as the button/ zipper issue would have been and Four: With three weeks of school left and a teacher's salary, I really don't want to buy maternity work pants.