The 20 year is actual delivery within your body: take a pill once and it gradually releases the correct amount at the correct time with an option to flush it out of your system as well.
Like the contraceptive implant? Without know a thing about drug delivery, it seems like it'd be pretty feasible already. There's probably a reason it's not done for more drugs, but I can't think of a good one. Perhaps the fear of someone pulling out 6 months worth of drug X from their arm, and selling/abusing it?
Accidental breakage and overdose would seem likely to be a concern, as well. Breakage has been observed in contraceptive implants, but the resulting changes in drug release have not, AFAIK, resulted in particularly significant health effects. However, with other drugs that could be a bigger risk.