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.