In a standard AD&D game, rounds are assumed to be about one minute long. The combat system round lasts about 10 to 15 seconds, during which a typical swordsman will make about three to six swings. Most characters and monsters can only make one effective attack in this time; the rest of the swings are feints, parries, or just for show.
Higher-level characters with multiple attacks are able to make more of these swings count. Parries are followed up with ripostes. Feints suddenly become attacks when an opening presents itself. With time and practice, a skillful swordsman can make every swing of his sword a potentially lethal attack.
Combat Rounds and Game Time[]
Combat rounds replace the one-minute round in normal AD&D combat situations. If you need to keep count, five combat rounds equal one normal round, and 50 equal a full turn.
To make things easy, the DM can rule that a battle, regardless of its actual length, takes one turn. Characters tend to spend several minutes checking on fallen enemies, surveying their own injuries, and making sure that the enemy has abandoned the field before dropping their guard. Unless time is an issue, this is reasonable.
Spell Durations[]
All spells with durations expressed in rounds last for the exact same number of combat rounds. Spells with durations measured in turns last for the entire battle. If a spell measured in rounds is in the middle of its duration when the fighting begins, the balance of its duration runs in combat rounds.
For example, a 5th-level mage casts a haste spell that lasts three rounds, plus one round per level, for a total of eight rounds. In the fifth round, the party gets into a fight. The haste spell lasts four combat rounds before expiring.
Everything else about the spell still functions on a round-by-round basis, as it did before. A cleric casting heat metal still inflicts 2d4 points of searing damage in the third, fourth, and fifth combat rounds. A wizard with a feather fall spell still falls at a rate of 120 feet per combat round. Remember, this is an abstract system; applying physics properties to every situation (such as the fact that the wizard with feather fall is now plummeting to the ground at a hasty 6.8 mph rather than the standard 1.4 mph) may reveal lots of facts, but it won't make for a better game.