Expect to spend most of a day getting to San Pedro la Laguna. The fastest I have made it from Guatemala City, where the Aurora Airport is located, is under four hours, but normally I expect five or six hours. From Tapachula Mexico, it can mean having to spend the night in Quetzaltenango, making it a two-day trip.
Tourist vans and chicken buses are the two basic modes of transportation for the traveler. Tourist vans generally have more cautious drivers, are safer, quicker, and more expensive. Chicken buses are more interesting culturally, more crowded, much cheaper, more accessible to thieves and more difficult if you have much luggage. Things that can affect how long it takes are traffic, road conditions, and whether you make good connections when you have to transfer. Be prepared for unexpected delays.