Press escape on your keyboard, go to setup>game rules. At the top it will say "Matchframes." the number you put there will be the amount of time in the match. 500 is standard. Assuming you have a PC you may also to "ctrl+M" to get to the gamerules.
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lol fail "ctrl+m" gets you to mods. Do "ctrl+G" to get to gamerules. sorry for the typo.