They're good for friendships for a long time - only after a significant period of time would a closer LTR work. Intimate duality requires emotional maturity, something that duals must first instill in each other. Quickly going into a dual relationship would be dangerous because both parties are just so different. Doesn't makes sense to most people until after a long period of time.

I think this process would take anywhere from several months to several years, if at all.