In my experience, it's actually longer than normal all things being equal. The tankless heater will start to heat up when it senses the flow. Still takes a measurable amount of time to reach operating temp so it becomes Heat-up + Circulation, instead of just Circulation...
it takes about the same amount either way with out a recirculation system.....as soon as the water is leaving the heat exchanger on tankless it is up to the set temp....