The longest lasting carto I used was the Echo cartomizer. I rotated 2 for a full month. They didn't even die or clog up, I just figured a month was a good lifespan for them. They make them for the eGo too, called the Echo-E cartomizer.
A "rebuildable" clearomizer would likely last longer. Something like a CE4+, Vivi Nova, T3, ect. You just replace the heads when needed.
The longest lasting is probably a rebuildable atomizer. You have to replace the wick and coil by hand, but the unit itself should last a good long time.
Good luck.