The package claims that you can see visible whitening after only 3 days. This claim is true. I've never whitened my teeth before but they weren't too yellow to begin with and I noticed the difference after 3 days exactly. However, I also have sensitive teeth and by the third day, the strips had increased their sensitivity. The instructions say that if your teeth are sensitive, start using the strips every other day (instead of every day), so that's what I did. The sensitivity stopped immediately after skipping a day. I used the rest of the package in a similar manner: using the strips every other day, sometimes using them two days in a row. The whitening continued just fine using this method. My teeth are extremely clean looking now, but not that fluorescent look some people get with over-whitening. They just look pretty and clean.
A few notes on usage. The strips are easy to apply, especially if you follow the instructions and put the bottom one on first. I definitely recommend, as the package says, to use them BEFORE you brush your teeth. The instructions say it will reduce tooth sensitivity but there's also another benefit: if you use them daily before you brush your teeth, you won't forget to do it AND you can brush off the gooey stuff that the strips leave behind on your teeth. Also, the strips are EXTREMELY sticky. They won't come off accidentally; they stay on securely. They are comfortable to wear. When you pull them off, make sure your fingers are dry so you can get a good grip to pull off the strip.