It is a nice site very close to the center of San Ignacio. After crossing the high bridge into San Ignacio, turn left past the police station and up the hill you will see a sign to turn left. Take that turn then curve right up a steep hill. You will see the museum and visitor's center there.
We stopped by about 5pm to get information and were told it only takes about an hour to see the site and it closes at 6. We figured for the $5 entrance fee, if we wanted to come back a second time, it would still be worth the money, so we decided to visit that evening.
Besides the site itself, there is a nice museum with information and artifacts which made the visit even more interesting. It helps outline the history (Cahal Pech was settled around 1000 B.C. and abandoned by 800 AD), and also includes a scale model of the site.
An aspect of this site that is nice is the elite residential section. There are a lot of nooks to explore and enough structure there to inspire the imagination to think about how it was like to live there.
For more information about Cahal Pech, visit this article by Jaime Awe