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