The main concept of Pokemon battle is to fight the opposition's Pokemon until his or her Pokemon is unable to battle (Which is usually determined by a referee). A time constraint is only seen in Contest Battles. In Contest Battles, there is a bar which decreases based on the appeal of the opponents attack more than the actual power. So in other words, it is no technically a HP bar. However, in regular Pokemon battles, a time constraint wouldn't technically be possible because:
- There's not actual HP Bar to determine which Pokemon has more health.
- Battles generally allow trainers to switch Pokemon and hence one Pokemon might start off with lesser health compared to the other and the time constraint wouldn't be fair while determining which Pokemon has more health left.
- Finally, most anime battles(Ash's battles), involve Ash having trouble in the start and then overcoming the difficulty and beating his opponent. This concept wouldn't work again with a time constraint
However, in the games, a time limit exists because trainers could potentially keep stalling, keep spamming the same moves etc in a repetitive manner causing the battle to go on for hours.