My house is built on a slab, and the plumbing is copper and under the slab. My kitchen sink, which is 50 feet away from the hot water heater has to run for 90 seconds before hot water appears.
This I understand may be normal because of the distance. What I am puzzled about is after the hot water reaches this faucet and is run for 5 minutes nice and hot, then turned off for only one or two minutes, and then turned back on, cold water appears and the hot water will not return and has the same delay. WHERE is the hot water going? Is it returning to the hot water heater?
When the house was built in 2001, the plumbers had opened the wall behind the hot water heater and explained the hot and cold were reversed and had corrected it.
I am currently thinking that perhaps during the time the plumbers discovered the hot and cold reverse problem, one of them swapped the dip tube from the cold to the hot inlet pipe thinking it was a quick fix and then did not correct it after they corrected the piping. This may created a backflow of the hot water in the pipe back into the hot water heater.The hot water in the pipe returning to the bottom of the hot water heater through the bottom of the tube.
Any ideas?

The very first thing I would check is to monitor the water meter after assuring that no water is being used in the house, no taps dripping, no toilets leaking. It the meter spins just a little bit, you may have a pin-hole leak in the hot water line in the slab.

suprised that the wall behind the water heater was opened to reverse hot and cold as the supplies outside the wall could be reversed and no big deal Yes as posted by others you likelk have a hot water line leak and remember in many cases just abandoned the leaking line and run a new one as easy as you can no need usually to follow the old line

If the hot water line was leaking, it would advance hot water to the leak and it would take less time for hot water to get to the sink. I would expect that your hot water line is oversized and uninsulated and is losing all the heat to the environment. It takes a while to get hot water to replace all the cold and lukewarm in the pipe.

You may have a leak in the cold water line which in turn creates a puddle under the slab. The hot water line (along with cold line) passes thru the puddle, losing its heat to the puddle on its way to the sink.

As soon as you turn the hot water faucet off, the hot water tries to heat the ground around the pipe and that cools it off.

Cold water under slab leaks are VERY uncommon. And as mentioned, a hot water leak works like a recirculaton line and keeps the pipe hot.

Convection, the heated water is transferring the heat to the soil, sand or whatever is surrounding the pipe.

A house built in 2001 would have copper installed under slab WITHOUT insulation and that would pass code?

Some contractors will do anything to save a buck