I keep tripping up on this! Asked to choose correct answers in a multiple choice, I chose as follows:
It takes 10 minutes to paint it = Se tarda..., Se tardaron... and Tarda...
Tarda was marked as incorrect. However, it seems to me that it has some legitimacy. Thing is, there's no indication what 'it' actually is, so it could in fact be the subject of the sentence! That might seem odd, but perhaps if you were to think of 'it' being a train in the sentence '(el tren) Tarda dos horas en llegar a Madrid' it might make a bit more sense - especially if this was said as a reply and the subject was already established.
If the question gave a hint like 'form a passive' or 'we don't know who or what the subject is', that would leave little doubt. But as is, it not entirely clear.