Hate to say this but there isn't one, and trying to stick to a specific amount of time isn't really the point. The question is
Is everybody getting what they want out of it?
I know that you, for instance, don't enjoy a session where you get 10 minutes of the spotlight in a three-hour session. So as your GM, I would want to shine that spotlight on you more than that...while someone else in the group might not like all the focus on them for that amount of time.
So the answer to your question is more complex and infinitely more SOCIAL than a simple time ratio. It's about knowing your players as much as anything else.
Everything is playing.
You answering the player's questions is important to the players, while you are simply impatient to "get on with the thing." Every moment should serve the fun and the story. You describing and clarifying the setting IS playing the game.
The fact that you think it doesn't and it isn't is a hole in your technique as a GM.
As a GM, I have trouble giving descriptions that don't prompt a lot of questions, cutting into time playing the game, vs describing/clarifying the setting...
Not a problem. An opportunity. There's no ratio except this: 100% of the time when you are the GM, you are there to serve the awesome, and if you want to turn it up to 11, it's about taking those times that you don't believe are worth anything and making them worth something, or moving on to something that is.