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How often for a high heat cycle?

I have a 40 gallon Bradford-White propane water heater with standing pilot. It is set for 145f with a mixing valve set for 120f. I've installed a side-arm (tube&shell) heat exchanger using the boiler water from my steam boiler. The thing works great but only gets the dwh up to 125f. It's plenty hot enough for my needs, but not hot enough to kill legionella.

I'm thinking the easiest way is to turn the propane on every so often and bring the tank temp up to 145f for 30 min-1hour. My question is: how often should I crank the heat? Once a week? Once a day?