It depends on the model, quality and features. The one that came with my current house was very finicky and needed a fair amount of handholding (including pouring water into a little tube at the start of the season - something about a proper vacuum). I replaced it a few years ago with a new, near-top-of-line Trane with an air filtration system and a whole house humidifier. This one only needs to have the filter vacuumed monthly (three-stage electrostatic filter), and to have the humidifier water panel replaced once a year. Otherwise it’s basically no maintenance.

Any forced-air system has ductwork, however, and that can get filthy over time and need cleaning.