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
Hmm, honestly it's a question that anybody who does something creative can answer, and it usually just circles around to finding a way to motivate yourself. They say motivation comes when you least want/expect it, and honestly it's true for me - like when I'm on the metro going to work, I suddenly get these cool ideas for posts or videos that I would "totally do if I had a computer on me", but then when I get home and I have a lot of free time I just have no motivation to do the ideas which I "was totally looking forward to doing".
If you look at it from a RP-only point of view though, it could also be that RPing is usually a pretty long-term commitment that you need to spare at least an hour or two for (if you're writing a decent-size post). You don't feel or consider any of that when you're having fun obviously, but when you lack motivation (the above) having a whole hour of writing ahead of you can seem daunting and I often find myself putting it off like it's a chore, even though I know that I'll enjoy it when I do eventually start.
time, motivation, and the procrastinating. if i get a lightbulb, i write. it all depends on what kind of post i'll be making, i usually have a reply popping out all of a sudden but i'm always too late to type it.
For me it usually comes down to my health which is poor to say the least. If I'm feeling healthy then I usually try to get as much done as I can in general, RP included. Then on days where I'm not so good I cut back and only do what I feel up to and know I'll enjoy - sometimes this means RP but usually not.
I always try to be responsive as a GM though, I consider that a responsibility I can only shirk if I'm really, really not well.
I need to be interested in where the plot is going and in my own characters in order to want to write. If I am, like in Capes 2 rn :D then it goes really easily and I can find myself planning out posts in advance during my day. If not, it is literally a chore to sit down and churn a chapter out.