Counting number of messages for a mailbox over several periods
Some suggestions ...

Use a counter cache for messages on Mailbox -- the count of all can be read from the mailbox instance.

Use #size instead of #length. This will use a counter cache for messages on Mailbox if one is present, or will use a SQL count. #length will get all of the messages back, then check the size of the array.

I would be concerned about having multiple mailboxes. If your user model has_many :messages, through: :mailbox you can run these queries more efficiently:

messages = current_user.messages

@today    = messages.today.size
@week     = messages.week.size
@month    = messages.month.size
@all_time = messages.size


With a counter cache, you're only running three queries to get the counts, and thy should be efficient enough.

Place a single index on the columns (user_id, id) on the mailboxes tables, and an index on (mailbox_id, created_at) on the messages table.

Edit: I just noticed the week and month variable assignments were round the wrong way. Also, if you wanted to be slightly more efficient you could define a scope for week_except_today and month_except_week, etc, and then:

@today    = messages.today.size
@week     = @today + messages.week_except_today.size
@month    = @week  + messages.month_except_week.size
@all_time = messages.size