Do consider that I am a middle-aged man with an artificial hip and can barely touch my toes. I can also kick at my own head-level with little effort. The reason is not flexibility in the hips, but the fact that I like to do standing abdominal crunches. This, plus the flexibility in my lower back allows me to get into position with truly mediocre hip flexibility. The point is that there are far too many joints and other anatomical structures involved in something like a high kick to be reduced to simple flexibility. On top of this, excessive flexibility can lead to a higher incidence of injury. I see karateka all the time trying to hold their legs up in static positions and a bevy of other completely useless drills. Oriental stretching methods are just not that good. Like yoga, it is that you do it for years rather than a clear-cut understanding of various anatomical issues. Of course, the fact that yoga is a lot of fun for some means they are much more apt to do that than something else.
Below I have a pretty good general body stretching routine. You get pretty much everything. These are compound stretches, meaning that you are doing several things at once — just like everything else in taiso. A good stretch should take you, by the clock, 3 – 4 minutes. This is not too much to invest. The stretches themselves naturally fall into subsets of 3 stretches. When you are stretching, hold each position for about 10 seconds then go onto the next position. The subsets are chosen for each of getting into related positions rather than anatomical function. The best time to do stretching like this is right after a good workout, when your muscles are at their warmest. The positions are aimed at the average person. Feel free to vary them and play with them. The point is to stretch rather than maintain form. Everybody is different.