Philippe;
I would start at 10 minutes but this will vary, of course, depending on your exposure srtyle and your developing style (i..e tubes, tray, brush etc). Sandy (King), I believe advises on something like 16 minutes to start for this combination but I found that to be way too much for me when printing on Azo. Your results will vary but I personnaly processed (old) 8x10 Tri-X for 10 minutes (+- a few minutes) doing one sheet at a time by brush processing. I would guess you would would want more than that for Pt/Pd however. Do a simple test with 5 identical sheets processed at 8 10 12 14 16 and see which works best for you.
Bonne chance, cousin Francais.