iPhone 7 Plus takes days to turn on showing Apple logo or black screen
...when my phone died I put it on the charger, then the logo appears
  and goes off. When it goes off, a black screen pops out with a little
  white line on in it, then it turns on back to Apple logo. It keeps
  doing this until it turns on which is 1 or 2 even now 3 days.


This is indicative of a battery failure and you need to have it replaced.  

Your battery is having difficulty holding a charge under a certain level (batteries are really never at 0V).   The battery itself has an operating range - it may be 12 to 8 volts DC.  Under 8V, it will give you the black screen and not power on until it reaches that minimum threshold.  If it's taking 2 or 3 days to meet that threshold, it means that the battery's capacity has diminished to a point that the charger can barely keep up with the dissipation - think "keeping your head above water" analogy.

There will come a point in time (very soon) where the charger simply won't be able to charge the cell(s) anymore.