Open an EMS window. First set a variable with the current date e.g.
$d = get-date
Then subtract the number of days you want to go back e.g. for 60 days
$d = $d.AddDays(-60)
Now we can list all mailboxes where the last logon date is earlier than 60 days ago
Get-MailboxStatistics | where {$_.LastLogonTime -lt $d}
If you want to restrict the search to particular servers or databases use the -Server or -Database switches to the Get-MailboxStatistics command.