The L1A's are good for more than 7 years for commuters who don't reside in the US as only days in the US count towards the time limit, which is 3 years and two 2 year renewals. As for pilots who moved to the US, most applied for green cards. So for the commuters from Canada that visa is at least good for 20 years. The only pilots who might get kicked off the US base when onshored are the Canadian FO's if CX can't get them a proper visa.