N-400 application questions

Ok, that sounds straight forward enough, so what about the option of postponing the filing. As per the previous question, does it matter if I do not apply at the end of the 3 years?
And if I do apply later, what are the requirements for physical presence in the country... i.e. would I have to be here for 2 out of 4 years, or 2.5 out of 5 years etc.

Thanks again for your time.
 
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