Arbor Day Foundation suggests pruning or trimming trees once per year during the dormant season, which can vary depending on the species of your tree. According to the University of Minnesota Extension, circumstances such as these may prompt an immediate trimming:

The tree's growth obstructs visibility for pedestrians or vehicles, especially at intersections.
The tree's limbs interfere with power lines. Make sure to contact your local utility company to handle the job, as it's simply too dangerous to even get close to power lines.
The tree's growth may threaten your home or property. If you feel a tree needs to be pruned because it may cause damage to your home, or causes safety concerns, it's important to contact an arborist to help with the trimming.