Its hard to say for sure. I believe Dr. Lee and Dr. Proctor have both said that spironolactone once/day is still effective - to a lesser extent.
spironolactone rapidly converts to weaker antiandrogens once it aborbs into the stratum corneum. I dont think anyone knows how long it stays active on the androgen receptors once absorbed, but I doubt its a very long time.
I think a person's response to spironolactone probably depends on the aggressiveness of their male pattern baldness. If you are a slow male pattern baldness progessor, then even using spironolactone once a day and giving your follicles an approximate 5-6hour rest from DHT binding might make a difference. If you are agressive, then you'd probably have to use spironolactone at least twice a day along with propecia. Remember, even propecia only reduces DHT by around 40% in the scalp but its still effective. It stands to reason that if spironolactone can reduce DHT binding by 70%(its affinity) for a short period of time, then it should have some beneficial effect, even when used once daily.
this is all my own rantings, i have no studies to back it up.