water, water, everywhere..but not a drop to drink
Unlike the other fluids coursing through your powerplant (gasoline and motoroil) coolant is NOT consumed in normal operation.
Coolant fluid is essentially moving in a closed loop, from the reservoir, through the radiator, waterjackets, waterpump, hoses, etc. The only openings in this system are the overflow and filler pipes on the reservoir. Fluid can therefore only be "lost" if there is a leak in the system, such as a damaged head gasket, waterpump gasket, or leaking hose. If you have to repeatedly add coolant to your system it is indicative of a potentially serious problem - get it checked out ASAP.
A couple of other coolant notes here - first, make sure you are checking the coolant level when the fluid is cool, and with the bike level. Secondly, absolutely DO NOT use automotive type anti-freeze in the cooling system. Auto anti-freeze contains silica (basically sand) that is supposed to scour out the insides of auto cooling systems - it also does nasty things to the guts of motorcycle engines. Use only motorcycle-specific coolant.