I typically will always have my 20' lanyard on my harness for most climbs. However, if I'm climbing a smaller tree, I'll use my 7' lanyard. Using the longer one can sometimes end up being cumbersome and organizing all of it after each use so that it doesn't get in my way can be annoying; especially if I've used close to or all of its full length. However, I like being able to use it as a second line to maximize comfortable work positioning and so that my secondary TIPs aren't limited to what's immediately around me, giving me more options and better angles. It's also reassuring to know that, in situations where I'm by myself, such as when I'm climbing recreationally, should my primary TIP or climbing line become compromised or fail, or should I somehow lose access to it, , and if I don't have spurs on (I can't just down climb the stem), I could potentially use my long lanyard to get down through the canopy using a MRS, pulling the rope down and connecting lower and lower until I'm at the lowest branch. At that point, if the ground is 10' or less from that branch, I could continue MRS or, if it's 20' or greater, I would use my prusik with a ring to connect my lanyard into an SRT setup and come back for all of my gear once I've been able to go back home and grab another rope. I'm not entirely sure how something like that would happen, but you never know, right?
Anyways, the rope length I use the most is a 200' line. I have two 75' ropes for small trees (I cut a 150' rope in half).