It's not an area where I have much practical experience, but I do know the theory pretty well! The purpose of flushing is to remove water that has been static. So the time should link to the flow rate & 'dead volume'. Once you have cleared the volume, continued flushing won't do anything more - there will be some biofilm on the surfaces and the best you can do is have water in the area that has been properly treated to minimise further growth. What you suggest for the mixer taps seems sensible. However, it would be better to choose an outlet in that area with no mixer, if one is available. You don't have to temperature check every out, just representative ones.

When testing at an outlet fitted with a mixer valve, you should use a flat probe to test the temperature at the pipe that leads into the mixer.

Oh, and forgot to say that removing stagnating water also should remove sediments and organic matter in the flushed area too. Not just about the water and bacterial load per se.

Thermostatic Mixing Valves (TMV) are separate from mixers taps and commonly used in hospitals etc. where there is a risk of scolding. These should be regularly maintained and tested to ensure that the temperature at the outlet is not above the limit to cause scolding (if I remember rightly 45 degrees C). The flushing of infrequently used outlets, as has been mentioned, is subject to being identified by the users of them. The length of time it takes to flush them through is as previously stated of approx. 5 mins at most. Additionally you must be aware of the potential exposure to the person doing the flushing as opening the tap full into the basin etc can cause droplets to become airborne. I would recommend a full water hygiene assessment be undertaken if the property is rented or falls into the category of care home etc. (it doesn't say what type of property it is in the original post). The assessment would look at all elements of the exposure and potential and would recommend actions to address potential exposure (a bit like a fire risk assessment works). Things to watch out for are the rubber flexi-hose fittings commonly used and also unused showers etc. We had a regular planned preventative maintenance (PPM) schedule that would address flushing and temperature readings. Temperature readings were taken at the furthest outlet from the heating source (i.e. boiler) and also at the return to the boiler and any adjustments made to ensure they were above the minimum limits. Additionally temperature readings were taken from any header tanks or loft storage tanks (also factor in the variance in temperature throughout the year). There is a lot to consider other than temperature reading and flushing.