The driers are full of beads or small pellets of what are called molecular sieve and these filter out water vapour from the system. Water gets in because it can diffuse in through the rubber hoses, then it can start and corrode from the inside out. Except for the Gen7, the cars I've had all had a sealed unit drier with the sieve inside, but the 7 must use some kind of re-fill pack?
An AC system in good condition needs a regas about every 4 years and I take the view that I'm happy to use a good professional, usually charging ~£50(?) to do it. A pro will take 30-45 minutes to do the job (anything less is worrying). Be sure to check that you get leak-indicating dye added.