This answer entirely depends on the scene and your and your partners' preferences. If you find yourself hesitating how much to include or how far to move things along, ask them if they have a preference. That is, I think, one of the best things you can do because everyone has different preferences. Talking it out between you can ease up on that uncertainty far more readily than anything else can. 

For 1x1s, I've noticed that as long as there [i]is[/i] something for the other character to react with, most people can come up with a post. And even if there isn't something directly in the post, provided you both know where the events are heading, they still have a direction to push the character in. But, as with most things, if they're stuck, talking about it is a great solution. Offering to edit your post or add something in is probably always appreciated. 

For me, I go by the rule of how slow and intimate do I want this scene to be (as in how much detail and character emotion and internal processes do I want to include)? And what is the scene's purpose? If you want to delve into those details, then you don't need to make a lot of stuff happen or even move things forward by more than a minute at a time. If you are writing in order to move things along or because it's fast action, then there's really no need to linger and you can roll on through the scene until your next slower moment. Unless your partner (or you) would prefer otherwise.

Another good rule of thumb to work with so that you can avoid that unintentional character control is that if you're writing and suddenly think that the other character(s) would definitely break in on the conversation or the action or just have a big enough reaction to whatever has been said or is going on that it might derail the rest of your post, that's usually a good spot to either pause and ask your partner what their character might do if your post feels toooooo short to post or just post it. As you write with someone and get to know their character(s), you'll more often be able to guess how they'd respond to various things you stick in your post, so this does get easier to judge between a casual reaction, like answering a simple question, or them suddenly throwing a tantrum because your character mentioned the wrong name... You've probably felt this already. 

Also, if it just starts feeling like an infodump, feel free to stop at the next feasible point or go back and take some out before posting to save it for later, because information is good, but too much at once will have something being missed and your partner possibly going cross-eyed. 

It's not a perfect science and most of us are probably at least a little concerned about this question no matter how long we've been writing. The key is to be sure that you are satisfied with the information you've conveyed, and that your partner is happy. If they're happy with the post, who cares if you think it was long or short or uninteresting or overly complicated? They liked it.