Although tankless units are often referred to as "Instananeous", there are multiple factors that will affect how quickly the water will produce the required set temperature.  Once the unit detects flow, it must go through multiple steps to ignite the burner and reach the required operation.  Generally the start up of the unit happens quickly, with water leaving the unit reaching the desired set temperature within 5-10 seconds.  The distance from the water heater to the fixture will also affect how quickly that water gets "hot" since that water must get from the unit to the faucet or fixture.