How long is too long? With our low-tension gut (or plastic) strung banjos, I should think that getting them securely tied to the tailpiece, and a clean, uniform wrap coming out on the base of tuner is the important thing. We don't change strings as often as the modern wire strung crowd, so what's the hurry?
I like to replace all my strings when I change out. Having two or three old ones (usually the forth, second third and second) with a fresh gut first and fifth seems to "unbalance" the instrument. That's why I spool as much as I can onto the tuner, especially on the first and second, and wind it down when it becomes frayed near the bridge.
When replacing a set of gut strings, I take them all off, and remove the tailpiece, so I can insert the string from the bottom. Tie each one on, then put the tailpiece back on, and start cranking. Takes about 15 minutes, I guess. Then an hour or so of stretching.
With Nyl-plastic, it takes a little longer to stretch, but when do you need to replace them? They will still be usable when we're all planted 'neath the sod ;^}