Fuses (FUSEWIRE or cartridge based fusewire) can wear out. They are designed to tolerate a particular current, but can handle a much higher current if subjected to it for a short enough time but it will degrade the fusewire over time.
You should replace fuse/wire and bear in mind that there may be a fault and look out for it, i.e. switch off all lights on the dodgy circuit before returning power and then turn the lights on individually to see if a particular light causes the fault. If the fault is other than a degraded fuse then it will present itself pretty quickly by blowing the replacement fuse and then you know you have a more serious fault. See theshedmans answer :)
Does the fuse blow when all lights on circuit are on? Yes - then too many lights, and/or too much current being consumed for that circuit.
Normal design (when I was in the game) is max 10 lights per circuit rated at (Av.) 100 watts per fitting, or 1 kW(ish) per 5A circuit.