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I changed it when I got that weird annoying "dried oil" thing that made it look like I had a scratch. Now its all good.
 
My favorite protectors are the matte, anti-glare versions that have a little 'grip' to my fingertips. My protectors ship in packs of 3-5 and are replaced after each loses that 'grip' - about three or four months.

Part of this is just because of my OCD, though. :D
 
Buy a pack of 50 cheap ones off eBay for 5 bucks - Change them when I get a single scratch.
 
you did it yourself? ... i am afraid i may get many bubbles trapped.

After going through about 30 of them, it's very easy.
Have some scotch tape on the side, clean with microfiber cloth to remove oils then tape your whole screen with one long piece down the middle as the bottom layer so you can remove the whole thing in one motion.
When you're ready, remove the tape (This will grab any dust on the screen, MUCH better than using a microfiber cloth) and apply the screen protector quickly. The tricky part is aligning the screen protector... can't really help you there. If you have any dust particles stuck on the screen, get some of the scotch tape, lift the screen protector carefully as not to grab more dust particles and use the scotch tape to grab any dust under there.

If you catch bubbles, use a creditcard and slowly press them out. As long as there aren't any dust they'll eventually come out.
 
Changed like 3-4 times on the first month of my iPhone ... And now i am using a matte guard which was installed 101% without any bubbles and gone use this until & unless i get a bubble & i can easily use this for next 6months without any issue.
 
I rarely change them. I usually purchase 2 really good screen protectors with the 2nd one as a spare. On my 4S I had a Power Support Crystal Clear and never changed it. Had the phone for 1 1/2 years before trading it in. My husband dropped his phone and put a pretty good gash on the protector. I talked him into changing it after a few months and also to change up his grossly yellowed (previously clear) TPU case. I would have changed it right off but I'm a little more anal than him.

I've also gotten pretty good at putting on the screen protectors. They even come to me at work for an install. Give me a steamy bathroom, a lint roller and some scotch tape and I'm good to go! At work it's a little harder as I don't have a steamy bathroom #. What I forgot to mention is that you need patience - do your prep work (super clean and dust free screen) - just take your time and go slow to get a perfect alignment.
 
For quality screen protectors (PowerSupport & SGP), I've kept mine on for at least a year.
 
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