What's a Reasonable Inbox Size?

Greetings,

I have several users who have inboxes that are over 20 GB.

Lately I have noticed Dovecot logs say it's taking over 30 seconds to sync their mailboxes.

As email admins,  how do you handle inboxes that are so large? Do you use mailbox types that have better performance like dbox? We're using maildir.

What's a reasonable inbox size?  Is 20+ GB reasonable and nothing to worry about?

Thanks for any insight here.

-- Asai7