You finish a commercial certificate somewhere around 250 hours. An airline wants 1,500, or 1,000 to 1,250 with the right degree. That gap is the single largest obstacle between a new commercial pilot and an airline seat, and for the overwhelming majority of civilian pilots it is closed by teaching.
The reason is arithmetic. Dual instruction given counts as pilot-in-command time under FAR 61.51(e)(3), and an instructor at a busy school flies far more than they could ever afford to rent. A full-time CFI in Southern California, where the weather cooperates 355 days a year, can log a substantial number of hours a month, and the usual estimate is that the gap takes twelve to twenty-four months. It varies enormously with the school, the season and how many students you carry, and anyone quoting you a precise figure is guessing.
The other reason to instruct is less commercial and more honest: you do not fully understand a maneuver until you have had to explain it to somebody who cannot do it yet. Instructors are better pilots than they were at 250 hours, and it is not because of the logbook. If the airline path is the plan, get the CFI certificate, and get it early rather than treating it as a fallback.