hi,
as I understand correctly that you have 2 level bom and each header and component can be sell,
and as you go up into the bom level then the manufacturing time gets increased.
in my opninion - you can take ptf for 1A1 -5, 2A2 - 5, 2B2 - 5, 2C2 -6, 3A3 -11 days, now it is also based upon your availabality of material and sales order requirement. also it depends upon you safety stock.
in my opinion it should not be two short that it has not much effact and at the same time it should not be too long so it will delay you delivery, for example for material 3A3 you can not take as 20, because it is too long.
you might get a better suggestion from a production planner of the comapny because he might have actual and realist views.
regards,
Solomon