“To find perfectly ripe fruit, catch it.”
― Khang Kijarro Nguyen
Some plants can be harvested all season and some are best harvest in batches.
Lettuce heads can be harvested over and over as needed. Just pull leaves off of the outside of the plant as cloes to the base as you can. The lettuce will keep growing from the midde out untill it begins to grow tall or put out stems and flowers. When this happens your lettuce is at the end of it's edible life. You can pull the leaves if you catch it early enough and see if they still taste ok. If they are sour or bitter, toss them. Because they go through thier life cycle faster than other plants its a good idea to grow these in batches. I start new lettuce every couple weeks so that new heads of lettuce are ready for the tower by the time old ones are ending their usefulnes.
Tomatoes should be picked in bunches when the fruit become ripe and before they split. You can pick slightly undreripe tomatoes when they are green and starting to show a darker blush. They will ripen off of the vine because they produce their own ripening gasses called Ethylene. Picking earlier when green will let your plant put energy to younger fruit and help from keeping the plant from becoming too heavy.