The clutch fluid should not go down by much at all (if any..) normally it just darkens and looks dirty because of the hot environment its forced to work in
My clutch master cylinder (the lever part) has just developed a moderate leak so i am going to pull it to bits and replace the seals, or worse.. the whole unit..
A more common problem is the slave cylinder (the bit at the end of the line thats bolted to the engine block, near the front sprocket) has a notoriously crappy rubber seal, it literally dissolves in the fluid..
Its a scary prospect that it would wear enough to let clutch fluid escape but its well worth checking how it looks
Another weak point is the bleed valves are made of relatively soft metal which leads to overtightening them, which can damage them to the point of leaking.. so have a good look at the threads where the bleed valves are
Because its a sealed system (supposed to be airtight..) i dont see how fluid should ever dissapear, its not like it actually gets burnt.. so i would advise against ignoring this one, find out where the leak is before it causes any more serious damage
Best of luck, hope its something simple!
Wolf