For me, it took 5 weeks of past papers to go from C to an A and another month to get an A+
If I was you it wouldn't be enough time for me to get an A, however I did got B 2 weeks after doing past papers.
No matter what you got 19 days left, just do whatever you can, you spend 2 years doing physics no point to not even try.
Maybe try to find past papers for which there are videos on youtube from teachers, so you don't spend too much time understanding the mark scheme they will tell you what to do (and comments usually help too), since time is limited.
My exact numbers are (I kept track):
9 past papers to get my first B
28 past papers to get my first A
After 40 past papers I kept getting A+ low A*
However I found physics to be the most challenging A-Level, for maths it took me 10 past papers and I got an A sooo you never know you might get an A.