Rate of Fenton Reaction
This is only a partial answer:

I am not sure about which step you're looking for or the substrate of your reaction, but the Fenton-based chemistry I work with is extremely fast. I use it to (supposedly) form a methyl radical from DMSO which then reacts with an α-iodoester to form another radical.

Literature suggests I add brine immediately after I've completed the hydrogen peroxide addition and work it up from there. Truth be told, I usually give it a few minutes, but the reaction is certainly quite fast. The whole process takes me less than 5 minutes, and if I wasn't using DMSO and skatole (so smelly), it would be my favorite reaction of the ones I do.

I use iron(II) sulfate heptahydrate as my catalyst, and I use 0.5 equiv. relative to the iodide. It ends up being about 0.1 g per milliliter of solvent.

As for the peroxide titration, I use a decent excess, so I've never needed to titrate it. That is why I said this was a partial answer. I will say this: don't quench large amounts of hydrogen peroxide with sodium thiosulfate at room temperature. You end up forming sulfuric acid in aqueous solution. Very exothermic (like the water in the flask can start boiling in your hand).