Why 90
Nathaniel Kleitman described a roughly 90-minute basic rest-activity cycle that continues through the day, and Ericsson's study of elite violinists found their focused practice happened in sessions of about 90 minutes with rest between. Neither is a rule for you, but 90 minutes is the longest block most people can hold at full quality, and the one that gets long, hard work done.
How to run it
Write the task first. Choose a low, steady sound: brown noise or wind. Do not plan to check anything at the halfway mark. When the chime goes, leave the desk for the whole break.
Kleitman (1982), "Basic-rest-activity cycle", Sleep, 5(4). Ericsson, Krampe & Tesch-Römer (1993), Psychological Review, 100(3).