A week in Antigua [1] would give you ample time to explore the town, its ruins, museums [2], and churches [3], maybe climb a volcano [4], visit a coffee farm [5], and do some shopping [6].
Depending on whether or not you plan to study Spanish [7], you could easily spend several weeks in Antigua. Some choose to study Spanish for a week just to brush up on their skills or get a very basic foundation before moving on to other parts of Guatemala.
At the minimum, you should plan on spending two nights here. Some have even recommended Antigua as a long weekend getaway from cities such as Miami, Atlanta, Houston, and Dallas because of its proximity and ease of access. The Guatemala City international airport is about a 45-minute drive away.
Getting around Antigua is fairly straightforward. True to its colonial foundations, it was laid out in a grid pattern surrounding the central plaza with calles running east-west and avenidas running north-south. The plaza is bounded by 4a Calle and 5a Calle to the north and south, and 4a Avenida and 5a Avenida to the east and west.
Street addresses are labeled according to their direction relative to the plaza: Norte (North), Oriente (East), Sur (South), and Poniente (West). Most streets are known by this method, though all have names dating to colonial times. Only a handful of streets are known solely in this manner.
Links:
[1] http://www.moon.com/destinations/guatemala/antigua
[2] http://www.moon.com/destinations/guatemala/antigua/sights/museums
[3] http://www.moon.com/destinations/guatemala/antigua/sights/churches-and-monasteries
[4] http://www.moon.com/destinations/guatemala/antigua/recreation/hiking
[5] http://www.moon.com/destinations/guatemala/antigua/near-antigua-guatemala/centro-cultural-la-azotea
[6] http://www.moon.com/destinations/guatemala/antigua/shopping
[7] http://www.moon.com/destinations/guatemala/antigua/language-schools