Here are some guidelines to help you decide when to harvest your vegetables:

VEGETABLE	HARVEST TIME
Beans, Snap	Harvest before you can feel the outline of the individual beans. If left too long, shell the beans and discard the pod.
Beets	For greens, harvest the inner leaves while young and tender. Roots should be harvested when no larger than a tangerine.
Broccoli	Harvest broccoli while the head is still tight. Once the buds begin to spread it will soon flower.
Cabbage	Heads are ready to harvest when they are firm and heavy. If left too long they will split. Split heads are edible, but difficult to deal with in the kitchen.
Carrot	Carrots can be harvested any time. Small carrots are the sweetest, but it's best to leave them in the ground until you need them so they won't dry out.
Corn	Once the kernels turn a golden color they are getting too old, and by the time a dent appears in the end of the kernels they are virtually inedible.
Cucumber	Harvest before the seeds inside swell.
Eggplant	Harvest when the fruit is small and glossy. Once they loose their shine they are too old.
Lettuce	Baby lettuce is a gourmet treat. Harvest the leaves as needed from the outside. Once the stems begin to lengthen and the leaves begin to grow up, the plant is getting ready to bolt and will be bitter.
Muskmelon	Pick up the melon. If the vine falls off it's ready.
Onion	Onions can be harvested at any stage. When the tops fall over they should be dug and dried for a day, then braided or stored in a net bag.
Peas	Harvest before the individual peas begin to touch inside the pods. They become starchy and tasteless if left too long.
Pepper	Harvest any time, depending on individual taste.
Potatoes	Potatoes begin to form when the plant blooms. Soon thereafter you can harvest small new potatoes, or wait until the plants begin to die back for large potatoes.
Summer Squash	The younger the better. Squash left on the vine too long inhibits the production of new fruit.
Winter Squash	Most winter squash should be left on the vine until the bottom is a different color from the top, and the skin is hard.
Tomatoes	It's best to wait until the tomato is ready to eat. A tomato that is picked when is is showing any orange or red color will be superior to a supermarket tomato.