How long a journey was it from Horeb to Kadesh-barnea by way of Mount Seir?

Answer: Eleven days

It was an eleven-day journey from Horeb to Kadesh-barnea, yet Moses gave this address in the fortieth year.

“It is an eleven-day journey from Horeb to Kadesh-barnea by way of Mount Seir. In the fortieth year, on the first day of the eleventh month, Moses proclaimed to the Israelites all that the LORD had commanded him concerning them.”— Deuteronomy 1:2-3 (BSB)