I run from Hayfield village where I live. Closest parking is at Bowden Bridge.
How long depends on fitness, I allow 2 hours. 3+ hours sounds reasonable.
The downfall is a most frustrating subject. At it's best, it's one of the most spectacular views in the locale, but more usually it's a pile of grubby rocks with a trickle of water coming down it. Spectacular water this morning, but rubbish light.
I've been up there a few times in the last couple of months and got nothing worth showing. I even planned a dusk shot hopping for that alpenglow effect, but sadly the haze in the west killed the light. My reward a 5 mile walk back to Hayfield in the dark.
The money shot is when it all freezes in winter, not that that has happened for a while.
If anyone fancies a wander up there give me a shout.