tests on finasteride showed that a least in alcohol water, I think it said it broke down 45% in about 2-3 weeks. A 25% water, 50% ethanol, 25% propylene glycol 0.025% finasteride solution was effective at regrowing hair in a 6 month study. I've not heard of any tests on it in minoxidil. We just know that spironolactone reacts with minoxidil, and caffine reacts with one of those two.
spironolactone has a funny oxygen triangle group on it that I think makes it reactive, not that I'm an expert on that, but that is just the feel I get after seeing a lot of reactions. I think water is the reason finasteride breaks down slowly.
I don't know much about the topical dutasteride. It is a lower dose than finasteride. dutasteride builds up over time, whereas finasteride works at its dose and is gone the next day. finasteride has less chance of building up in your system if you are worried about side effects, though at low doses it does not have much of a half life.