It can rain all day, and it can snow for hours on end, but hail always seems to fall for short periods only. Why don't you get prolonged periods of hail?
Hail is created within a thunderstorm by updrafts that carry water droplets to an altitude where temperatures are lower than the freezing point for water (32°F or 0°C). This freezing process forms a hailstone. This process can repeat itself each time increasing the size of the hailstone until the hailstones become too heavy to be picked up again.
The creation of hail requires high wind speeds that may only be sustained in the storm for a brief period of time. Longer periods of hail are possible but typically require large strong storms such as tornadic supercell thunderstorms.
The first reason is that hail is intimately connected to extreme updrafts in violent, high-energy, high-wind storms, mostly supercells. High-wind storms by their very nature tend to move quickly, so they go as fast as they came. And for obvious reasons an updraft must be a local phenomenon (or the air in a region would go away). One can have multiple hail storms on any given day when the conditions are right, but each one is necessarily a local phenomenon.
So we need to be more precise: Hail storms are short at any given location, but the storm can be longer lived and create more hail elsewhere, together with tornadoes, hard rain and other short-lived whether phenomena. Sometimes we can see a cell move over land from a distance. We can see that rain or hail continue to fall out of it, but the cell is constantly changing location.
The second reason, concerning a somewhat longer timescale, is that the condensation depletes the storm of energy which must be replenished, or the storm dies eventually, as can be seen with hurricanes: They are long-lived only over warm bodies of water which serve as the energy source. Over land they abate quickly.
This would make it possible that there are longer-lived hail storms over the sea, but I have no data about that. A remark on theweatherprediction.com says that hail in hurricanes is rare because the air is so moist and warm (i.e. energy laden) that it freezes too late and too far up.
The other answer is correct: the mechanism of hail formation is the hail repeatedly travels up the cloud, gets heavier, and then when it becomes too heavy, it falls out of the cloud. This process has a number of preconditions such as an updraft and a lot of water, and when those preconditions are disrupted, so is the hailstorm.
But I want to look at it from a different perspective. Here, let's do an exercise. Find a fridge with an icemaker, and extract ice from it as rapidly as possible. If your icemaker is like mine, you can get a huge amount of ice out for a very short time, and then you wait a long time for more ice to be made.
There is no lack of moisture in an icemaker; the plumbing supplies as much as it needs. And there does not need to be an updraft, obviously. But in both the hailing cloud and in your icemaker, there has to be some process which is extracting heat from the water and moving it elsewhere in the environment. And that process takes time and energy. Once all the stored ice is gone, you're going to have to wait, and you're going to have to spend energy to get more.
You have to extract 80 calories of heat from each gram of liquid water to freeze it regardless of the current temperature and pressure of the air. And it takes an extraction of a whopping 540 calories per gram of water vapor to turn it to liquid, again, regardless of the temperature and pressure. That heat has to go somewhere! We think of hail as a massive exportation of coldness from the cloud, but you can't get coldness somewhere without putting that heat somewhere else.
That heat energy that is somehow extracted is going to change the characteristics of the cloud. And there are real physical limits on how much time and energy it takes to transfer that heat somewhere else in the cloud.
Your icemaker has a basically unlimited supply of moisture and perfectly stable conditions for making ice, and once its reserves are gone, it cannot supply a rapid flow of new ice. Even if a cloud had an unlimited supply of moisture and perfect ice making conditions, it also cannot supply a rapid flow of new ice once the reserves are gone. It turns out that it is really expensive in energy terms to make ice out of water vapor, and so on energy grounds alone we would expect hailstorms to be short.