Geostationary satelites are essentially for ever. This is becoming a problem since there are a limited number of places you want to put a geostationary satelite and most of them are full. Any collisions/explosions in geostationary mean debris will also stay there for a long long time.
For low earth orbit satelites it depends on their shape, altitude and the space weather.
The worst case is a large object in low orbit with large solar panels and hence a lot of drag. The ISS loses 90m/day and must be constantly boosted as the orbit gets lower the drag is worse and the falling accelerates - when first launched the ISS would lose 500m/day at it's lower orbit. The minimum safe orbit is around 150km, anything that falls to this level will quickly succumb to atmospheric drag
edit: Hubble is at an orbit of around 600km, with no more service missions after the end of the Space Shuttle it will renter in between 10 and 20years, ie 2020-2030