In general, other respondents hit most of the points. I wanted to add that fillers in the lips (mainly the HAs) won't last as long as the same filler placed in the nasolabial folds or cheek area. The longest area of duration is around the eye as several docs have suggested.
The theory is where there is more muscle movement, there is faster degredation. I find the nasal injections last about the same time as the nasolabial injections, maybe slightly longer.
There are also rapid metabolizers of HAs. For unknown reasons, some people chew up the filler faster than the general population. I've also heard of some rapid Radiesse metabolizers discussed at some meetings. If you think you're a rapid metabolizer of HAs then try Radiesse.
A new filler called Evolence will be FDA approved very shortly. It is a cross-linked collagen and will be labelled as lasting about 6 months but will probably get approval for saying 12 months after more studies are submitted. It can't be put in the lips, although a second form of it, called Breeze, will be able to.