But I don't think I've ever spent time to learn a particular word - it's almost always enough to hear it in context once, and maybe get a chance to actually use it yourself once or twice, and you'll probably remember it for life.

If it's a word for a more complex concept (e.g. some mathematical construct), you may well need more time to actually understand the meaning, and you may also pretty easily forget the meaning in time, but you'll likely not forget the word itself.