How do I get the Time information after STFT?
How, can I find the change of frequency with Time (900000 sample) or 15 min?


OK, so you need 900000 frequencies.  Well, one way to do that is to take each element of your 637 length vector and expand it to (up to) $H$ samples.


  Moreover, how does changing the window length and overlap affect the outcome of STFT? 


The longer the window length, the better the frequency resolution is. The shorter the length, the worse the frequency resolution will be.

Before we can answer your question, we will need to know what form your signal takes.

If it's a single tone in noise, then you may be able to use one of these or one of these or this approach.