You could take the time before and after the refresh with VBA, and assign the difference to an user variable.
Add the following code to the ThisDocument module of VBA. Refresh the document and you will have a variable called <Execution Time> with the time between before and after refresh.
[tt]
Option Explicit
Dim BeforeRefreshTime As Date
Private Sub Document_BeforeRefresh(Cancel As Boolean)
BeforeRefreshTime = Now
End Sub
Private Sub Document_AfterRefresh()
Dim ExecutionTime As Date
Dim FormattedTime As String
ExecutionTime = Now - BeforeRefreshTime
FormattedTime = Format(ExecutionTime, "Hh:Nn:Ss"

On Error Resume Next
DocumentVariables.Add "-", "Execution Time"
DocumentVariables("Execution Time"

.Formula = FormattedTime
End Sub
[/tt]