@Nildog : some areas deduct dues once a month, some weekly (in my Local, it's 25% of the monthly amount taken out of the first 4 paychecks in a month. When there's a 5th payday in a month, no deduction). Also, sometimes there could be "catch-up" deductions being made if the company or union made a mistake, or as my Local Union requires, paying up 2 months in advance (there's a reason mine does this).
TL/DR: Could be either monthly or weekly. Ask a knowledgeable co-worker (not management) or union shop steward, or call/email your Local Union's dues department (or TITANS operator-that's the name of the software most Locals use for dues accounting) for a breakdown of the dues/fees payment schedule.