I don't use a mono bumper, so I change the lower few feet often, mostly based on visually inspecting it, if I see any fraying or damage at all, OFF comes 6' or so, tie on the swivel/snap, back to fishing. As for the entire spool of line, I don't really change them on a schedule, I stick with judging the appearance, if the color goes OFF or it starts to look bad, time for new line.
As far as trimming OFF a couple feet every season that is what I have been doing. I have about 3 yrs on what I have now it has faded some but isn’t frayed. Seems strong I will probably look at changing it next year. Thanks for the input.
the other thing I might do with your example is reverse the line onto another reel, so the older but still good line goes to the back, and fresh line comes to the front. I've done that now and then with the 60# braid on my level wind chinook reels.
I read an article that said most braid lasts about 3 years with care and if you reverse it onto another spool then you could get another 3 years. I always figure better safe than sorry. I don't want to be the one that loses a trophy fish because I was too cheap to replace my line when it gets close to it's time.