depends really. it's not good practice but if the engine is quite high mileage and to be polite, not a particularly high tech unit, then the correct quantity is the important thing. obviously it's better to change the oil regularly and try and keep to the same brand. but, if it's the same grade and type then no harm done.
Mixing grades is OK (but you end up with a strange garde that's neither one thing nor the other), but you mustn't mix mineral, synthetic or semi-synthetics.