Honestly it just depends on the build, and when that build is "finished". A lot of builds go to 50 because it is a nice balanced level that allows for everything you want. Some people don't need that much, numerous builds cut off at 40 or 45. Even lower can work if it is for a specific purpose - ie. Alastor's Vancian Wizard is done at 30. Like I said at the beginning, it is just whatever works for the build.
I guess maybe I do this backwards. I really don't pick a level. I decide what skills I want, about how far I figure they will get leveled and then figure out how many perk points I have to work with (using the attached chart and some guess-work). I figure that speech and lockpick will always get leveled, but count those together as one skill, unless one of them is going to be significant to the build. I generally find that I use between 4 and 6 skills which puts me in the mid 30's to mid 40's for most builds.
That said, if I can't find a use for all of my perks, I have no problem stopping early. I have one build that is only a level 18 build, even though I could eventually max out two skills (and thus have 24 perk points).
I have never messed with making skills legendary, because I find some of the fun to be figuring out how to make the build viable if the perks I want outnumber the perk points that would be available.
Interesting. Personally I'm a fan of lower-level builds, 40 is my general max these days but I usually prefer to end at 35. I did used to enjoy playing upto level 50 but now I feel that working on a build for an extra 10-20 hours can beat playing an extra 15 levels.
It really depends on the character though, some character just beg to be allowed a higher level, and others beg to be put out of their misery (Sometimes I tend to find that perfect ending point, where the character has hit it's pinnacle)
Though I will never play an uneven build, 37, 33 or even 34 is just a no go, it has to be divisible by 5 (yes even if that means I play an extra 2-4 levels) just something that I can't physically do.
I guess a lot depends on the character and what you wish to present, as others have said. There is a lot to be said for pick up and play builds, by that I mean the ones with a tight focus that can be experienced in a weekend. Some of Paul England's builds are great at this as they are inspiring and accessible, giving the player the experience intended by the author. The Elf Hewer, The Clever Man and The Swordthane are good examples of this. So in those cases I see nothing wrong with level 30 or 40 builds. I did a Crystal Tower build and set my representative perks as level thirty. I got to much higher levels, but felt 30 was good because by then I had completed all my objectives and assembled all the pieces.
On the other end of the spectrum there are the epic builds which require a huge investment of time but allow the player to reap huge rewards. Many of Mason's builds are like this, the Heretic and The Sentinels being good examples. I think I recall him saying that for him rp didn't really start until the builds were complete.
So I think it depends on the experience you are presenting. There is nothing wrong with either extremes, most prefer to walk a middle ground.
I guess I differ a lot from Mason there. Builds have to have enjoyable RP and gameplay right from the start for me to be interested. I hate having to hide behind followers or sit in towns grinding to pay for training while 'setting up' for a build. I want to be out there, punching dragons!