Set Booking Window for all Room Mailboxes with powershell.

$session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri “https://ps.outlook.com/powershell/” -Credential $cred -Authentication Basic -AllowRedirection 

Import-PSSession
$session 

get-mailbox|where {$_.recipientTypeDetails
-eq “roomMailbox”} | Set-CalendarProcessing -BookingWindowInDays 365

To check the windows:

get-mailbox|where {$_.recipientTypeDetails
-eq “roomMailbox”} | Get-CalendarProcessing | FL Identity,BookingWindowInDays