It depends upon the crop when you remove the cover, why you are covering and what kind of cover you are using. I'm keeping the potatoes covered with Remay until they flower, then will re-cover when done flowering.

Yes, you really time off of last frost.

Dan

Most of the summer crops have the seedlings started so that the planned planting date is 2 weeks after last frost. So, for a planting date of 5-15-10, you would start seedlings on the following dates ...
Peppers (10wks ahead) 03/06/10
Tomato (8wks ahead) 03/20/10
Okra (6wks ahead) 04/03/10
all of the following 4 weeks ahead ...
Cucumber 04/17/10
Watermelon 04/17/10
Cantaloupe 04/17/10
Honeydew 04/17/10
Yellow Squash 04/17/10
Zucchini 04/17/10
Beans * Direct sow outdoors on Planting Date
Cowpeas * Direct sow outdoors on Planting Date

Of course, the spring crops are planted before the last frost date such as peas, lettuces, broccoli, carrots, beets, etc.