Every other day to start with, then twice a week when I was starting to get on top of it, down to once a week as a maintenance thing for the rest of the winter (bearing in mind he lived out, so was never actually getting off the wet ground). 10mins at a time. To my shame I didn't catch the thrush soon enough so it was already fairly bad when I started using the Milton - as Raf's doesn't sound too bad yet I'd say soaking once or possibly twice a week would be enough, and use your other preferred products in between (I rate the Red Horse products, they worked a treat for us!).
Yes my farrier likes the purple spray. I have had good results with mine using it.Make sure feet are thoroughly picked out when he goes out & comes in, and make sure you clear all around the frog properly. Ensure your bed is clean & dry, and deep enough that the wet drains away leaving the top of the bed dry. I like Antibac to treat & prevent, but a friend who has a horse very prone to it has good results purple spraying clean dry feet 2 or 3 times a week - get the proper stuff, not the watered down rubbish sold in tack shops.