A stare of how many seconds is a decent stare?

Is there a decent way to stare at girls without seeming creepy ?

Usually I stare at girls that I find attractive. Some girls look away, some return back my stare and some give me the look of WTF are you looking at ?

I tried my best to stop staring at girls but when I do, I get frustrated throughout the day with no apparent reason o_O

Also, is it wrong to stare at girls ?
A stare of how many seconds is a decent stare?
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