Kilometers that can be driven on different base oils
-Mineral based oils have a potential to last for any way between 3000-5000km,
-Semi-synthetic can last for 7000-9000km,
-Paraffin based and synthetic based oils can easily last for 15,000-20,000km
But this all depends on:-
-The temperature of the country,
-Kilometers covered in a stretch,
-The time for which your car is stagnant.
This all is mentioned for cars.
And for visual checks
-You should check the oil levels on the dip stick,
-And as mentioned by 'mindcorrosive' the colored and the viscosity can tell you the condition of the oil. The darker and less viscous the oil gets you know its time to change.
-Change the oil when the engine is a comparatively hot, helps to drain the oil easily but be careful not to damage the threading on the oil drain nut.
-Change the oil filter with every oil change.
-If metal filings are found in the oil you have a big problem, similarly if less oil is drained out.