Powershell: Wait after Enable-Mailbox until mailbox is ready for further commands (Exchange 2013)

I'm adding a ressource mailbox to a new AD ressource (user) and configure it after with the following commands (as you see I'm handing over the domain controller server name that I don't have to wait for the AD syncronisation):

$mailbox = Enable-Mailbox $pUserid -Database $mailboxStats[0].Name -DomainController $pDcServer -Room -ErrorAction Stop

$mailbox | Add-MailboxPermission -User XX\XXXXX -AccessRights FullAccess -DomainController $pDcServer -ErrorAction Stop

$mailbox | Add-ADPermission -User XX\XXXXX -Extendedrights "send as" -DomainController $pDcServer -ErrorAction Stop

$mailbox | Set-CalendarProcessing -AutomateProcessing AutoAccept -DomainController $pDcServer -ErrorAction Stop

The configurations are just an example of the different commands I'm using to configure the mailbox after creating it.

I had to wait up to 300 seconds after Enable-Mailbox until the Set-CalendarProcessing command returned successfully, before I alway get the following error:

Cannot open mailbox /o=XXXXXXXXX/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Configuration/cn=Servers/cn=XXXXXXXXX/cn=Microsoft System Attendant.

Is it really needed to wait that long that the mailbox is ready? Is there a possibility to get rid of the waiting time or to know when the mailbox is ready?