Personally I don't take any supps every day continously, ie: years, except vit c. It's just my personal opinion but I do it to keep from any types of 'build up'' or overuse of said supplement and let my body clear itself.
I found a couple extracts on PubMed about DHEA studies but I think the summary on LiveStrong summarizes well:
http://www.livestrong.com/article/326567-the-best-sources-of-dhea/
For this discussion I agree w/ G-man - most studies don't go beyond 3-6 months, so we don't know. However general rule of thumb on any supplement would be to cycle it. I think in all things possible, "Supplement" your body and allow it a cycled phase to work w/ what you've been supplementing or potentially improve accommodation of the supplement's impact w/o the supplement itself constantly being there. The basis for this, IMO is the fact that the body is built on an intricate collection of push/pull systems - everything is regulated by the body to produce as well as to flush out both the main components and the by-products of anything in the system. For example, a high protein, restricted-variety competition diet - sure, a targeted diet will promote a dramatic change in bodyfat / muscle development - but at the cost of excluding a more balanced diet that includes fruit & vegetables that help the body break down certain foods, anti-oxidents to help shuttle out the waste from processing a high-protein diet.
It's a very broad generalization, but based on the general way the body works to be as self-sustaining as possible. That said, its probably also an interesting experiment to try for yourself and see what you notice or don't notice.