Currently, on live for Protection Paladin, the baseline duration of DSteed is 5 seconds, 7 with Seasoned Warhorse, and 9 with Unrelenting Charger. On PTR, the baseline duration is now 3 seconds for Protection. There are no class change notes regarding a change in duration for Divine Steed, so it should still be 5 seconds for PPals.
Edit: I had War Mode on. Oops.