How long does the whitening with trays last?

There's no easy answer, it really depends on the individual. After whitening, try to avoid things like coffee, cola, tea, grape juice, red wine, cherries and cranberries. All take home tooth whitening systems have basically similar active ingredients (often carbamide peroxide) with the differences being flavorings, thickeners, etc. As far as trays go, you want ones custom made for you by your dentist, not generic ones you can get at a drug store. With higher concentrations of the active ingredient there does seem to be more sensitivity, this is the main side effect of whitening. There is probably a lower incidence of side effects with dentist office whitening products vs. over the counter ones because office patients are screened first and monitored, and treatments are customized to each person. The systems used to whiten teeth only work on tooth structure, not crowns, veneers, onlays, etc. The only way to whiten them is to replace them. Keep in mind that after whitening you should use a whitening toothpaste at least part of the time and that your whitening should be touched up about once a year.

The length of time it lasts can vary depending on your habits and how dark your teeth were when you started. For example, if you lightened several shades there is going to be some rebound. If you lightened only 1 or 2 shades, you won't notice that as much and thwe results may last a year or so. I always put a shade/color that we match with our porcelain shade tabs in the patient's chart so we can compare over time. Once our teeth are lighter, we get used to them and don't realize how white they are when we think they are getting darker. This way you can tell objectively and whiten accordingly.

The answer to your question is very difficult because it depends on a lot of factors including the character of your enamel and whether your diet includes things like red wine and coffee.  Give it a try and see how it works for you.  Good luck!