For how long does a deep cleaning(scaling or root planning) take at the dentist office?

It varies: The length of time for treatment depends on how many teeth are being treated and how much surface calculus is present and how inflamed the tissue is.

Four appointments: With deeper pockets, advanced periodontal disease and extensive rough root surfaces, the deep scaling and root planing might be broken down into four separate quadrants. The average time per visit (one quadrant) is 45-60 minutes.