I think two weeks may be something to do with the UK.
In France the deadline that exists (in most services) is a 48-hour deadline prior to the strike for individual union members to report whether or not they plan to participate in a strike that their union has filed. The 48 hours then allows management to come up with the alternative minimum schedule.
Unions therefore have to file their strike motions officially sometime before 48 hours, so their members can file their individual participatory decisions by 48 hours. But I don't believe there is any other rule/requirement (certainly no two-week minimum that I have ever heard of).