For people diagnosed with or being assessed for high blood pressure, a common approach is to measure twice in the morning and twice in the evening for 5 to 7 days, discarding the first day, and bringing the average to your doctor. Once your pressure is stable and controlled, less frequent monitoring is usually enough.
For an otherwise healthy adult, occasional checks (for example at a pharmacy or annual review) are reasonable. Measure while seated and rested for five minutes, feet flat, arm supported at heart level, and avoid caffeine, exercise and smoking beforehand.
Constant checking can cause anxiety and misleading single high readings. Keep a simple log and discuss trends, not one-off numbers, with your doctor or pharmacist. They can also check your monitor is accurate and the cuff size is right.