For most people the sweet spot is a block of 45 to 90 minutes of uninterrupted focus, followed by a genuine break. Ninety minutes lines up roughly with the body's natural ultradian energy cycle, after which concentration tends to dip no matter how disciplined you are.
If you are just starting, do not aim for 90 minutes on day one. Begin with 30 to 45 minutes, protect it completely, and extend as your focus stamina grows. One reliable 45-minute block a day beats a heroic three-hour session you only manage once.