Definitely on the high side, but I'm guessing they mean "chronic abusers" rather than just "user". The serious ketamine addicts I knew when I was younger took the drug every single day, and multiple times during the day. I think that figure seems right, since a single large line can be north of 200mg.
I know: it's insane that someone would do that much ever, let alone every day, but I assure you that it happens.