The ranges from data sheets
Bambu Lab prints a retraction range on the data sheets of its filaments. Its PLA Basic and PLA-CF take 0.6 to 1.0 mm, and its PETG-CF, TPU 95A and PAHT-CF take 0.8 to 1.4 mm. Every one of them asks for a speed of 20 to 40 mm/s.
The retraction tower
OrcaSlicer’s retraction test prints a tower whose retraction length grows with its height. For a direct drive extruder it starts at 0 mm and ends at 2 mm, in steps of 0.1 mm. For a Bowden tube, OrcaSlicer suggests starting at 1 mm and ending at 6 mm, in steps of 0.2 mm.
Look for the lowest section that prints without strings. Measure its height, then open the G-code preview at that height and read the length from the Calib_Retraction_tower comment. The lines marked ; retract are not the length you want, as OrcaSlicer points out.
After retraction
A tower can look stringy for reasons that retraction does not fix. Wet filament strings more, so a spool that pops and sizzles at the nozzle may need drying first. The filament drying chart has each maker’s temperature and time.