Whoa I was way off. Sorry I was sort of traveling.
https://wiki.project1999.com/A_ghoul_assassin
https://wiki.project1999.com/A_ghoul_supplier
Assassin is 20% Supplier is 25%.
but masks are 80% and dirks 20% so
1/ (.2 * .8) means stand to gain is mask per 7 spawns rounding up, lets round the time up for people fucking around, 1 mask per 4 hours. Whatever. Just camp that shit.