High tide itself is an instant, but the water stays close to its peak for roughly an hour either side of it, about two hours in all, and NOAA puts the whole fall from high to low water at six hours and 12.5 minutes. A tide table prints high water as a single time because it is the moment the rising water stops and begins to fall. What people usually mean by the question is how long the water STAYS high enough to matter, and that is the hour either side: the water moves slowest around the turn, so the level has changed little before then.
The rule of twelfths, and why the ends are slow
Over the six hours or so between low and high water, the rise is not even. The old rule of twelfths is a decent approximation: in the first hour the water rises about a twelfth of the range, in the second two twelfths, in the third and fourth three twelfths each, in the fifth two and in the sixth one. That means half the whole rise happens in the middle two hours and the first and last hours together account for a sixth of it. Around high water itself the level is therefore nearly flat, which is what makes the hour either side feel like it lasts.
How long does high tide usually last in practice
If you are launching a boat off a ramp that only works near the top of the tide, the useful window on a typical coast is about two hours centred on the printed time. If you are walking a flat at low water, the same arithmetic gives you about two hours around the low. On a coast with a small range the window is longer in practice, because the total movement is small; on a big-range coast it is shorter and the middle of the tide runs hard. The range is on every place page here for that reason.
When is high and low tide on the same day
On most of the US coast the day holds two of each, alternating roughly every six hours and twelve minutes: high, low, high, low. Each is about 50 minutes later than the equivalent event yesterday, so the sequence walks forward and a calendar day sometimes shows three events rather than four. The only reliable way to know today's order at your beach is to read that place's table, because the tide arrives at different times along one coast and inside a bay or a river the turn can lag the open coast by an hour or more.
Other questions people ask
How long does high tide last for?
About an hour either side of the printed time on a typical coast, so roughly two hours near the top. High water itself is an instant; the water moves slowly around it, so the level stays close to its peak for that hour either side, and for a shorter period where the tidal range is large. NOAA puts the full fall from high to low water at six hours and 12.5 minutes, and successive high tides 12 hours and 25 minutes apart.
How long does slack water last?
Slack water is the pause in the FLOW rather than in the level, and near the turn it commonly runs for twenty minutes to an hour. It does not always coincide with high or low water: in a channel or a river the flow can keep running for some time after the level has turned.
When is low tide today, and when is high tide?
It is local to the place. Find your place in the index and its table gives every predicted high and low water for today and the next 30 days, with the height of each and the NOAA station the prediction was read from.