In Nathria normal with 14% haste and shield throw legendary/Kyrian, I averaged 60% SoTR Uptime. I offtank, and a lot of fights either have adds to offtank or a fast swap, so keeping it up is important. Also I like to maximize damage and skipping skills to conserve HP would lower that a lot.
I got some better gear, and I am now 19% haste unbuffed. I swapped legendary to the one that gives 5% haste, so 24% as long as I am fighting. This gives me 90%+ SoTR uptime, and I assume the 10% is due to my relaxing when not taking damage, etc.
Now on Heroic, I switched talents and went to Venthyr, and with Ashen Hallow allowing HoW throws, HP generation is easy. So with 24% and rotating Avenging Wrath. Ashen Hallow, and Seraphim to allow for extra HP generation, you can have 100% uptime if you try.
With that said, I still take 1.8X melee damage over my Monk tank counterpart. The only fight that it seems to be an issue is Hungering Destroyer on Heroic- a melee hit from him without SoTR is like 40% HP.