Edit/Edit - It took me almost a month of almost nightly whitening to get to my desired shade, so in total of 6 syringes of 10% carbamide. I'm too lazy to share before/after photos. I'll stick with buying this product for all maintenance whitenings going forward. I'm going to finish off the last two tubes to see how close to optic piano-key white I can get with 8 syringes.Edit: After an initial 5 nights of whitening my teeth were pretty white, but part of this was due to the teeth being dehydrated (and thus more sensitive). When you take a break for 1-2 days the teeth yellow a bit as the teeth rehydrate. So I started on another regimen, and am now on the 5th night of the second round, with 0.33 in 1 tube out of a total of 4 syringes used over 14 calendar days. Now I want to keep going to see if I can make my teeth look like piano keys.These work very well in your clear retainers. No need to purchase a separate custom whitening tray just for this. Check out the YT videos on how much to actually apply to each tooth-- it's just a small bead needed to the front of each tooth. I don't express a whole line of gel across the arch, this would be too much product.Also be aware that this is a perishable product. When I received the package, it had a remaining shelf life of 6 months. After, the gel loses efficacy. It doesn't drop to zero after this date, but just something to note. Amazon comped me an additional 8 syringes at no charge when I asked about the short shelf life.Start with buying 2 packs / 4 tubes of 10% Opalescence carbamide. I bought 4 packs / 8 tubes and there's no way I can get through all this stuff. I used only 1.75 tubes doing upper and lower arches for 6 nights and got to a very nice white shade, and any more my teeth would start to look like piano keys. I would've had to start from a much darker yellow shade in order to actually use up 4 tubes and go through two whole weeks of nightly whitening. And perhaps my teeth are more sensitive than others-- Definitely glad I started with 10% carbamide and not anything higher. By the 4th or 5th night I was starting to notice increased sensitivity, despite me having brushed with Sensodyne multiple times a day, every day for over a month prior. I also had braces removed a week ago, and experienced zero sensitivity during that procedure, for reference.