It takes between 40 and 90 days (depending on the season) for a lettuce to grow ready for harvesting.
Farmers use special machines called Harvest Rigs to harvest wholehead lettuce. Tractors guided by GPS pull the Harvest Rigs up and down the fields, and people walk behind them to cut the lettuce head away from the root. Some of the rigs can even pack the lettuce into a bag and seal it so it doesn't need to be packed by hand.
Baby leaves, like spinach and rocket, are very delicate and can't be harvested in the same way as wholehead lettuce. Farmers use a special baby leaf harvester which uses a band saw to cut the leaves at the bottom of the stalk without damaging them.