Goa is a delightful place to be with friends, and that is what I did. You need atleast 3 days if you only want to be at the beach, and a week or so, if it is your first time and you want to tour around the city, the fort, and the churches as well.
Though the afternoons are hot and can tan and sweat you, but the mornings and evenings are the perfect time to be at the beach and chill as per your own taste, in the shack, in the sun, or in the waters. The shacks in Goa beaches and generally cheap when compared to food and beverage prices in Delhi, which adds to the comfort of any budget traveler.
The more attractive and fun beaches are in the North, namely, Calangute, Anjuna, and Arambol, mostly clean and filled with lots of tourists, both Indians and foreigners. Evenings are beautiful on the beach with inviting reggae or country music playing in some shacks, and you MUST spend a couple of hours drinking and eating from the varied menus of the shacks. Everybody minds their own business at the beach, and its a fun place to be at when in a group. South Goa beaches (Colva etc) are less attractive (or so I found) and I would not recommend visiting them if time is short. (3 days)