Anytime a bit of prose---or poetry---comes to you and wants to be written down is The Right Time To Write that bit of prose. 

 Because, in my experience, if I don't write down something that comes to me as it comes to me (or at least repeat it to myself until I get to where I 
can write it down) it vanishes from my mind by the time I get around to setting it down.
But in my own work, beyond that, and beyond "writing what I feel like writing", the story-in-my-head-that-wants-to-get-out is so vast in its scope that I'm putting off most of the down-to-brass-tacks writing until I've gotten a lot more of the background work (outlining, creating character histories and profiles, mapping, etc.) done because I don't want to end up contradicting myself on something big (like having a character in two places at once).
On the other hand, as I say in my signature below, I fear I've developed the worst qualities of both Tolkien and Charles Williams: Tolkien's insistence on getting it 
right and tendency to fiddle with background details, and Williams' difficult prose and obscure allusions. So "Your Mileage May Vary."