There is no way in Pronto Hex to specify duration.
There is no way in the IR signal itself to distinguish duration that has been forced by the firmware from duration that occurred due to a long press of the button on an original remote or on a learning remote that properly learned the repeat pattern.
I'm sure you know that a learning remote does not record the duration of a long press.
The Pronto Hex at the top of this thread has no repeat part. The original signal probably had the same repeat part as an NEC1 signal, but was learned badly to produce that Pronto Hex.
Without a repeat part in the Pronto Hex, nothing tells the Pronto how to correctly extend the signal when a duration is specified. I don't know what it does in that case: Not extend it? Incorrectly extend it? Somehow guess right and correctly extend it?
But BEST CASE it correctly extends it: You still have an ordinary signal (like a long press of the original remote) from which another learning remote will NOT learn the duration.