Generally, patterns will tell you how long each section should be. When I knit a sweater, for example, the directions ususally say how long to knit to underarm. I generally like them longer, so I add however many inches I need.
I guess in my viewpoint, anyway, and I'm a little lax about this stuff, width gauge is more important since you can't adjust for that as you complete a pattern. If you find your length is not what the directions say it should be, you can always add more to it, or subtract if you find it too long. Of course for a blanket or scarf, it doesn't make any difference, you just keep going until it's as long as you want it.
So much depends on the yarn and stitch pattern, though, so each thing is individual.