We are doing a cross forest migration and move mailboxses from Exchange 2010 to 2013.
We have done the new-moverequest with the option SuspendWhenReadyToComplete(over 30 days ago).

Now when we tries to do a Resume-moverequest, but we get TransientFalure on all moverequests we have tried.

Is it true that the SuspensWhenReadyToComplete expires after 30 days (Deleted mailbox retention time)?

Hi Mortorp,

Thank you for your question. 

In official document, it didnt indicate that the SuspensWhenReadyToComplete expires after 30 days, The SuspendWhenReadyToComplete partameter is not recommended to use in Microsoft, we suggest you use the CompleterAfter parameter.

If we get transientfalure when we use resume-moverequest, we could enable the MRS proxy endpoint on Exchange 2013 by the following link, then run resume-moverequest to check if the issue persist:

https://technet.microsoft.com/en-us/library/dn155787(v=exchg.150).aspx 

If there are any questions regarding this issue, please be free to let me know.

Best Regard,

Jim

Use the Get-MoveRequest Command to find the existing move request. You may try to Remove / Clear the Existing move requests using Remove-MoveRequest. Then Preparemoverequest and New-MoveRequest.
Try to Restart the Mailbox Replication Service in Exchange 2010 CAS Server.
Ref to: Remove or Clear Move Requests

Here is an article about how to manage failed mailbox move requests.

Cross-Forest Mailbox Move

You may also take help from this earlier discussed thread - Mailbox move request error

Moreover, while need to migrate user mailboxes from exchange 2010 to exchange 2013 you may try this automated solution which lets you to accomplish this migration task without any interruption.

Hope it helps you!