Short and frequent is the useful direction
The Health and Safety Executive does not prescribe one legal interval for display screen breaks. Its practical example is that five to ten minutes each hour is better than twenty minutes every two hours.
That is guidance for changing activity, not a demand to exercise for ten minutes. Walking to refill a glass, standing for a call, or doing a short mobility burst can all interrupt a long spell in one position.
Use the work as the trigger
Natural boundaries are easier to follow than an alarm in the middle of a thought. Try the end of a meeting, sending a piece of work, putting the kettle on, or waiting for a download.
If your work has no natural changes, place one reminder near the point where you usually lose track of time. One reminder that works is more useful than six you dismiss.
Change activity, not only posture
A standing desk changes posture, but standing still is still one fixed position. Use some breaks to walk, climb a stair, reach, squat to a comfortable depth, or move your shoulders through an easy range.
A simple starting pattern
Pick two dependable gaps in tomorrow's diary and attach one or two minutes of movement to each. Keep that pattern for a week, then add another break only if it still fits.