The proper amount of oil is critical to the happiness of your engine. Too little and it may overheat, cause excessive wear, and in extreme cases (but not unusual), cause engine parts to seize and fail - very expensive. Make it a habit to check the oil regularly. Doing this after about every 5 hours of operation is generally recommended. More frequent checks - such as before each time you mow - are fine as well. A typical small engine in reasonably good condition does not use up a lot of oil but checking oil is easy and will not hurt.
Oil should be checked when the engine is cold or after waiting 10 minutes for it to drain back into the oil sump after running the engine.
If the oil level is high - you just bought the lawn mower or were careless in filling it last time - drain enough oil to bring the level back down to the full mark. Too much oil can result in problems as well - oil spraying out of various orifices or getting into other places where it should not be like the combustion chamber.
If you find the oil level over the full mark or higher than it was before, gas may be leaking into the oil due to a flooded carburetor - a stuck inlet needle or bad float. If this is the case, the oil will need to be changed once the underlying cause of the leakage is determined. (This is only likely with float type carburetors such as those used on the Tecumseh engines used in a variety of Sears/Craftsman models equipment.
If the oil is very low and you have been performing regular maintenance, there may be a leak or your engine may need a ring job. Excessive oil under the deck - on the shaft or blade adapter - would indicate a bad bearing or oil seal. Noticeable blue smoke while running would indicate that excessive oil is getting by the rings into the combustion chamber.
Typical oil capacity is just over 1/2 quart (usually about 1-1/4 pints).