As soon as you have finished the content. Finish all of them in a 5-7 days. I did it in 3 but did not do much apart from past papers.
This tells you how good your revision is going. You should improve past paper after past paper.
You also have to do past papers in the right way.
Lets say u done the textbook stuff made note etc.
You do your first paper get a B. Make sure you go over every mark you dropped and if there is a question you completely got wrong, oull the etxt book out and re learn that topic.
Next paper you get a A like 64/75
Don't say oh thats good enough, go over every mark you droped as before.
Sometimes you may not udnerstand how to get the correct solution, if so ask on TSR. There are some pretty smart people on here who are willing to help you with everything.
After a while u will get annoyed even with a score of 74/75. Don't drop any marks at home, go over all of them. In the exam you are bound to make some silly mistakes etc.
Only after you have done every paper like this should you do one past paper every week.
TL
R start past papers as soon as you have finished the material, and do them all close together with the mindset listed above.