Honestly, there is no right or wrong answer with regards to how white your bleach your teeth. More importantly, how white do you want them?
How much of your teeth show when you smile? What color is your skin? If a female, do you wear lipstick, and if so, how dark or light?
The good news with tray bleaching is that progresses slowly, usually getting your teeth a little whiter each time you do it. And then after you are done you might see a slight loss in the whiteness over a two week period where the color of the teeth bleaching is being normalized.
So, my suggestion is keep bleaching until you think your teeth a just slightly whiter than you like, and then stop the bleaching for two weeks. If after the two weeks, you change your mind and still want them whiter, than add another week of bleaching.
There are some older concepts than dentists used to use before cosmetic dentistry became so popular. The theory that the teeth should match the eyes no longer holds much weight with cosmetic dentists. The sclera (white part of the eye) gets darker with age and nowadays, most people want to look younger. The second theory that has fallen into much disfavor, especially among the lay public, is that if your are having crowns, veneers or bonding, the older you are, the darker the dentist makes the restorations. Again, almost universally patients want younger (whiter) looking teeth!