When will flying or hovering cars become reality?

whoa

I don't think flying cars will become mainstream to replace ground cars completely. It will be difficult to cultivate an economy for flying cars because keeping something aloft against gravity takes tremendous energy, and if you are just using a flying car for luxury, that's a lot of wasted energy, so only the rich will be able to sustain these for personal use. For Military use they might be more prominent because the government knows how to sink our tax money into things. There is a very fresh technology out that uses a special silicon wafer supercooled by dry ice, that allows the unit to levitate in mid air above a track of very strong magnets, the process loosely quantum locks the supercooled wafer in orientation using pin-like magnetic waves. A slight nudge causes the unit to move about the track with only air acting as friction. It has not been demonstrated that altitude can be manipulated at this point, so I wouldn'tt call it a flying car.

Probably never, with reservations.... The problem with flying cars is that it takes a lot of energy to keep them up in the air. So, that's the main physical restriction. Engineering a car that flies is not that great a problem - we can already manage to make them, and the costs of doing so are not that exorbitant. (Saw a demonstration of one a few years ago, and drove it. moving backwards and forwards once in the air was easy, but turning took a lot of thought, and landing was not that easy to do safely.) Most average human beings really only think/react in two dimensions, while flying requires thinking/reacting in three dimensions, and doing the delta v for turning is not intuitive - if you rely on instinct and intuition, you crash! So, as at the beginning: probably never!

We already have road cars that can drive themselves. We already have flying cars. Ish. Replace existing infrastructure? Maybe never.

Whenever someone invents them!

Not before the world ends.