A toilet normally will keep the same water level for days or weeks, depending on the humidity in the bathroom, unless a problem exists with one of the toilet's various parts. The presence of water on the floor near the toilet will let you know a leak of some sort is the cause, but other problems can also drop the water level.
The float triggers the fill valve in the toilet's tank, letting the fill valve know the tank needs more water. You can adjust the float level by bending the arm holding the float or by twisting the screw on top of the float if the float is the type that travels up and down on a metal rod. The float should allow the fill valve to fill the tank with water to about an inch below the overflow tube's opening.
A toilet tank can leak from a few different spots, lowering the water level in the tank and lessening the toilet's flushing power. The flapper on the bottom of the tank, which opens to let the water out of the tank, will wear out over time and allow water to pass down the drain unobstructed. The bolts and washers that hold the tank to the bowl can also wear out and allow leaks as can the toilet's fill valve. The way to determine the location of a tank leak is to drop food colouring in the tank's water and watch where the food colouring goes, pointing to the leak.
The toilet's trap pipe sits just below the bowl and has a curving shape to keep water in the bowl. The toilet's trap is made of porcelain, which can crack easily if subjected to hard blows, sudden temperature changes and other harsh treatment. Cracks in the trap will allow water to leak out, dropping the water level in the toilet's bowl as well. If the cracks are bad enough, they will let enough water out of the bowl and trap so that sewer gasses can pass freely into the toilet's bowl, filling the bathroom with offensive and dangerous gasses.
The water level in your toilet's bowl will drop over time if you have a siphoning problem in the toilet's trap. Siphoning takes place when a piece of toilet paper sits clogged in the toilet's colon in such a way that it continuously draws water out of the trap and down the drainpipe. The siphoning of the toilet water happens slowly over time, but it can lower the water level in the bowl enough that you need to flush more than once to wash down all the waste in the bowl. Using a plunger or auger will remove the partial clog and stop the siphoning.