In C:\Program Files (x86)\Steam\steamapps\common\Sid Meier's Civilization VI\Base\Assets\Gameplay\Data\GameSpeeds.XML, you can edit what each gamespeed's length is.
Making a game 3x the length of marathon:
Change this
<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>
<Name>LOC_GAMESPEED_MARATHON_NAME</Name>
<Description>LOC_GAMESPEED_MARATHON_HELP</Description>
<CostMultiplier>300</CostMultiplier>
<CivicUnlockMaxCost>300</CivicUnlockMaxCost>
<CivicUnlockPerTurnDrop>30</CivicUnlockPerTurnDrop>
<CivicUnlockMinCost>60</CivicUnlockMinCost>
to
<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>
<Name>LOC_GAMESPEED_MARATHON_NAME</Name>
<Description>LOC_GAMESPEED_MARATHON_HELP</Description>
<CostMultiplier>900</CostMultiplier>
<CivicUnlockMaxCost>900</CivicUnlockMaxCost>
<CivicUnlockPerTurnDrop>90</CivicUnlockPerTurnDrop>
<CivicUnlockMinCost>180</CivicUnlockMinCost>
and change this
-<Row>
<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>
<MonthIncrement>180</MonthIncrement>
<TurnsPerIncrement>100</TurnsPerIncrement>
</Row>
-<Row>
<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>
<MonthIncrement>120</MonthIncrement>
<TurnsPerIncrement>300</TurnsPerIncrement>
</Row>
-<Row>
<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>
<MonthIncrement>60</MonthIncrement>
<TurnsPerIncrement>170</TurnsPerIncrement>
</Row>
-<Row>
<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>
<MonthIncrement>24</MonthIncrement>
<TurnsPerIncrement>201</TurnsPerIncrement>
</Row>
-<Row>
<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>
<MonthIncrement>12</MonthIncrement>
<TurnsPerIncrement>129</TurnsPerIncrement>
</Row>
-<Row>
<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>
<MonthIncrement>6</MonthIncrement>
<TurnsPerIncrement>180</TurnsPerIncrement>
</Row>
-<Row>
<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>
<MonthIncrement>3</MonthIncrement>
<TurnsPerIncrement>264</TurnsPerIncrement>
</Row>
-<Row>
<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>
<MonthIncrement>1</MonthIncrement>
<TurnsPerIncrement>156</TurnsPerIncrement>
</Row>
to this
-<Row>
<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>
<MonthIncrement>60</MonthIncrement>
<TurnsPerIncrement>300</TurnsPerIncrement>
</Row>
-<Row>
<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>
<MonthIncrement>40</MonthIncrement>
<TurnsPerIncrement>900</TurnsPerIncrement>
</Row>
-<Row>
<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>
<MonthIncrement>20</MonthIncrement>
<TurnsPerIncrement>510</TurnsPerIncrement>
</Row>
-<Row>
<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>
<MonthIncrement>8</MonthIncrement>
<TurnsPerIncrement>603</TurnsPerIncrement>
</Row>
-<Row>
<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>
<MonthIncrement>4</MonthIncrement>
<TurnsPerIncrement>387</TurnsPerIncrement>
</Row>
-<Row>
<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>
<MonthIncrement>2</MonthIncrement>
<TurnsPerIncrement>520</TurnsPerIncrement>
</Row>
-<Row>
<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>
<MonthIncrement>1</MonthIncrement>
<TurnsPerIncrement>792</TurnsPerIncrement>
</Row>
-<Row>
<GameSpeedType>GAMESPEED_MARATHON</GameSpeedType>
<MonthIncrement>0.33333</MonthIncrement>
<TurnsPerIncrement>468</TurnsPerIncrement>
</Row>
Make sure you back up the config file before you do this, in case it doesn't work as you expect.
(If anyone wants to help with formatting to make this easier to read, feel free to comment)