How long is a match, and how does it start?

• A match is 40 minutes: two 20 minute halves with a half time break. There is no time off for injury. Local competition and tournament conditions may vary the duration.
• When time expires, play continues until the next touch or dead ball, and then the referee signals end of play. Any penalty awarded in that period is taken.
• Captains toss a coin with the referee present. The winner chooses the direction for the first half, the interchange areas for the whole match (including any extra time) and which team starts with possession.
• The match starts with a tap at the centre of the halfway line. A tap is taken by placing the ball on the ground at the mark, releasing both hands, and gently tapping it with either foot (or touching the foot on it). The ball must not move more than one metre and must be picked up cleanly.
• After half time the teams swap directions, and the team that did not start with possession restarts with a tap. After a try, the team that conceded it restarts with a tap.
• A tap cannot be taken until all attackers are onside and at least four defenders are onside, unless the referee directs otherwise. Delaying the tap is a penalty at the centre of the halfway line.

Source: FIT Playing Rules 5th Edition, definitions and rules 7 and 8.