Who has had lettuce nudibranchs?

[Delphinus]

On two separate occasions I tried the lettuce seaslugs (apparently they're not true "nudibranchs" .. I have no idea what the difference is, though). Unfortunately both times they just slowly got smaller and eventually disappeared. I think the longest one lasted maybe two months. I had since heard (from word of mouth) that this is not an uncommon experience with them, so I figured I'd never try that again. But now I hear of another guy who's had them for well over a year and actually had them multiply?? So I don't know what to believe.

Does anyone have any good idea what is a "normal" lifespan of these guys and has anyone else heard of them multiplying in their tanks?