There is literally NOTHING I hate more in an app than intrusive modal popups stopping me from getting done what I actually WANT to get done. Unless I really love the app otherwise, they usually generate an urge to insta-delete the app.
Badly written multiple choice polls can get as low as a 10% response rate, but well written one (which are optional) can get as high as 70%- to the user it can be fun if written well.
What we're learning is that length (brevity is king) plays a bit part of it, as well as speed to load the question.. we had to change our loading order to get better response rates for our users.
Also we recommend to put questions at the points that makes sense... eg while waiting for a game level to load, or after a big activity is complete and it's a natural break point...
Other point is - we typically recommend questions only go to a small subset of the audience - enough to answer the question you're trying to ask. So most users never see a question...