Given the range of computers on which Excel is likely to be running that seems like a complete non-starter. There will always be cases that take longer than X no matter what you choose for X.
Yeah, why not display a "Exporting..." dialog then that the user themselves can cancel if they think it's taking too long? Extending the timeout to X won't solve anything.
When you press Ctrl+V, the target window is focused. Forcibly switching back to Excel to show this dialog seems weird and confusing to me.
Then why not show a system dialog similar to copy/move files dialog after e.g. 5 seconds of waiting for a copy-to-clipboard operation?
Actually, now that you point out that this could be done, I’m surprised it wasn’t.