Some examples in order to see if I understood
1. Он долго одевается = He is dressing for a long time (based on what you said). Here I would have bet that it means: "He takes a long time to dress"
2. Он давно работает здесь = He has been working here since a long time ago. Here I suppose it means: He has been woling here for a long time.
3. Он давно жил там = He had been living there for a long time
Ya got it
(Except the last one).