How often are the blimps fully deflated of their helium and then refilled, and is all that helium just basically let up into the atmosphere and wasted? Given s

How often are the blimps fully deflated of their helium and then refilled, and is all that helium just basically let up into the atmosphere and wasted? Given some of our recent helium shortages, it seems really wasteful, even if it may be (currently) not costly.