iPhone 7 Plus takes days to turn on showing Apple logo or black screen I’ve had this issue for months but in the last couple of days it’s got very very bad.

When my phone runs out of battery it needs like 2–3 days to turn back on. At the beginning of this problem the phone would need about 1h. Every time when my phone goes off again the time to switching on goes up.

What does this all look like? Well, 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.

I tried to restore it and also update it but nothing happens, literally nothing happened.
...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.