Watering with manure slurry dilute - how often?

I saw on 'the wartime kitchen and garden' that they watered crops with a manure slurry - how often do people do that? Only once a year?

I have limitless access to horse muck and this year really trying to fix my allotments issues lol

I have had a google but can't seem to find anything, a firsthand account would be great?