Given Blizzard's track record, they seem to mass ban every six months or so. They will ban in between, but usually that is reserved for severe offenders.
Sad thing is, a ban doesn't even phase the serious botters. They will have a new set up and running in no time back at it on fresh accounts and IPs.

The only thing Blizzard can do is improve Warden to detect bots and autobans, but the bots keep better at avoiding it. Doubt they will ever truly eliminate botting as it is too costly to do a real permanent solution.