I wrote a post about gladiatorial combat basics and style which provides a good preamble. I'm also on mobile so please forgive any formatting errors.
There was no truly standard length to a gladitorial match. Depending on what you were viewing, be it the hunts, matched pairs, executions, or the rare special events, fights could last anywhere for a few seconds to an hour or so. Naval battles would tend to last the longest as they had to flood the Flavian Amphitheatre to do so they used them for more entertainment, the midday executions lasted the shortest individually as prisoners were either marched unarmed against animals or against each other with both being inexperienced fighters.
Matched pairs would last as long as the men could fight, those in good condition could last for up to 15-20 minutes at the longest. These were generally highly skilled individuals and were the most 'honorable' of the contestants as they fought in solo combat and with minimal protective gear (amounts varied from class to class).
Especially when the opponents were well matched in experience and armament style, the bouts could last as long as the men could fight. There was no hard line system of matching strong opponents against other strong opponents, sometimes an experienced fighter would engage a newly trained gladiator. Especially in the matches where gladiatorial styles were mismatched and the skill and experience such as the retarius pitted against a secutor or murmillo, a very common match, was needed to make a good fight. An inexperienced murmillo against a retarius would likely lose as would the reverse as the experience of how to fight the particular class of gladiator was crucial in the mismatch of speed and reach vs armour and power.
The editor had a vested interest in making strong matched pairs especially towards the end of the games as it would entertain the crowd and whose thirst for bloodshed had been dampened by the earlier executions and perhaps lesser skilled bouts. Better to risk two skilled and valuable gladiators at the end of the games when the crowd was less likely to demand blood. Overall the matching system was moreso to pair styles together with historic significance, as to whom the gladiator class represented, or historically prominent class pairings known to provide skillful combat.
The editor, the one who organized the fights would want to match experienced gladiators together to provide more entertainment while lesser experienced gladiators would either be a sacrifice to the crowd, matched against a better opponent, or against one of their own approximate level to provide the entertainment of a lesser skilled but, matched dual.
In the end the editor and sponsor had a vested interest in providing skilled opponents and quality duals as it would provide better social recognition for the sponsor and better economic benefit (both in displaying their skill in organizing a series of quality games and the price they could demand for their skills for future clients).
Sources: Cruelty and Civilization: The Roman Games by Roland Auguet (discusses the cultural significance of the games in Rome)
Roman Histories by Cassius Dio (includes descriptions of gladitorial combat through the imperial age)
De Re Military by Vegetius (mostly about late Roman military but does discuss gladiators somewhat)