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

You can use the "contact us" link at the bottom of any page to send a note to FL support. 

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