1. Not all glass is the same. Some glass is of a much better quality and therefore harder to etch. I use Armor Etch and it is usually sufficient to leave the etching cream on for no more than five minutes. Recently I tried to etch a glass cookie jar and it took about 45 minutes to get a proper result.
2. Do not remove your stencil until you are satisfied with the results. I learned this the hard way with the above mentioned cookie jar. After waiting five minutes, I rinsed off my etching cream and peeled off the contact paper. My image was visible, but just barely. It looked like I had used a razor blade to scratch the image into the glass. Since I tore my stencil when I removed it, I had to make a new one. Believe me when I tell you, it is nearly impossible to fit another stencil over an existing image, especially when you cut your stencils by hand.
3. The larger the image etched into the glass, the better the result. Small images do show up but not as well.
4. And last, but certainly not least... Etching cream can be reused! There is no need to wash it all down the drain. Simply use your application brush or a small silicon spatula to wipe off the excess cream and return it to the pot from which it came. Much friendlier on your wallet, not to mention the environment. Take care not to accidentally wipe it onto the uncovered glass, though. This stuff etches faster than you think.