Time + Motivation sounds about right, but I think there are a few more factors for me.
Interaction is a major factor in that if I'm waiting to see whats happening elsewhere in the RP, maybe those events could affect my character. Not so much their decisions, but events around them.
Another one is where the post falls on the narrative timeline. If it's a climactic piece, where the character overcomes some great obstacle or defeats the bad guy, then I like to spend a little more time on it or wait for the inspiration to hit me. If it's just build up, or a character or world building post, I can usually settle for what I come up with since making it up on the spot isnt too hard
Sooo, for clarity's sake:
Time + Motivation + Social + Narrative Tension
If at least two of those are in place, I can usually compromise and pull a post out in two days minimum depending on the last factor