I thought the longest a plate of superfood would go is 2,5 hours.  Mine is over that

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I have it that it’s 2.45 max. mid. plate…..

From the "Help" page:

Food TypeMax Time Between Visits
Regular Food (5 Wildlife in 18 hours)6 Hours
Organic Food (5 Wildlife in 12 hours)5 hours
Superfood (20 Wildlife in 24 hours)2.5 Hours

What does 2.5 or 2,5 hours actually mean? Two and a half hours or 2 hours and 50 minutes?

two and a half hours

The gap is not between one critter being spotted and the next one arriving. The gap is between the time the previous critter would have left, and the next one arriving. This can make it look like the gap is bigger than it is.
For instance, a field mouse can stay for up to 60 minutes. If it is spotted after only 5 minutes, there are another 55 minutes before it is due to leave. It is only after that 55 minutes that the 2.5 hour gap begins. So you can see that this might make it look like the gap is longer than it should be.

Ok, the gap thing makes sense. My plate of food went about 2 hours 35 minutes. I had never seen it go that long before

The longest times a plate could go, are not officially the longest times.  They’re times that have been put together by players from observation. 

So what does this all mean then please...:) :)

Food Feeding Times
 
  REGULAR single/5 feeds/18hrs
  1st feed MIN 1hr 33min
  1st feed MAX 3hr 35min
  Mid-Plate MIN 1hr
  Mid-Plate AVERAGE 3hrs 30mins
  Mid-Plate MAX 6hrs
 
  ORGANIC - single/5 feeds/12hrs
  1st feed MIN 33min
  1st feed MAX 1hr 53mins
  Mid-Plate MIN 30mins
  Mid-Plate AVERAGE 2hr 25mins
  Mid-Plate MAX 5hrs
 
  SUPERFOOD 20 feeds/24hrs
  1st feed MIN 3mins
  1st feed MAX 1hr 10mins
  Mid-Plate MIN 5mins
  Mid-Plate AVERAGE 1hr 12mins
  Mid-Plate MAX 2hrs 45mins