What's a Reasonable Inbox Size?

[Asai]

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