Unless your house is 30,000 square feet and the furthest faucet is 100 yards away from the water heater....you're talking 10-15 seconds at the absolute "worst plumbing job ever" case for hot water. Just turn the water on and wait... Recirculating a on demand water heater is about the dumbest thing I see on Reddit from time to time....

If you really see the need then put in as efficient a standing tank water heater as you can then recirc that. It'll be much better for the world and for your gas bill since that on demand uses a hell of a lot more gas when its running all day every couple seconds than a standing tank already insulated and hot. Not to mention you are going to chew thru that "MUch" more expensive on demand heater running it near continuous... to save you waiting a few seconds to get hot water at a sink...