Hey, sorry for replying to a long-dead-thread, but I can also blur my vision and did some research on it as well. I hope my research provide you guys with some useful information. Intensity
I found out that you can adjust the blurriness of your eyes. Putting it to the Maximum blur (The Max blur your eyes can handle.) Will make you have a double vision.
Time
I can blur my eyes for days, but that usually leads to muscle fatigue.
Possible Benefit
If you can control the blurriness of it, can you reverse the effect so that if your eyes become naturally blurry, you can reverse it and see things clearer; Basically, seeing without glasses. [I have yet to test this out since I am a person with 20/20 vision]
Some Notes
When you blur your eyes in an environment with an above average light level (Depends on the person.) You will feel pain in your eyes. [I don't know exactly about this since I am used to the dark.]