When should the tea timer start?
Choose a clear starting point for the timer and use it consistently. With very short infusions, the time spent filling and pouring matters too.

Choose a starting point you can repeat
The leaves begin meeting water before the kettle has finished pouring. That seems obvious, but it explains why two people can follow the same three-minute instruction and produce slightly different results. One starts the timer with the first splash; the other waits until the pot is full and the lid is on. Either routine can be repeatable if you know which one you are using.
For an ordinary mug infusion, starting as the water reaches the leaves is a simple default unless the product's instructions specify otherwise. Put the timer within reach before picking up the kettle. Do not try to operate a phone one-handed while pouring hot water. A few seconds of tidy preparation are more useful than an extremely precise display that makes the actual handling awkward.
The alarm marks an action
When the timer sounds, remove the infuser or pour the tea away from the leaves. Turning off the alarm and leaving the basket in the mug does not end the infusion. Have the drip dish or serving pitcher ready so that finishing the brew is a single comfortable movement. If a call interrupts you, note the delay rather than treating the longer cup as evidence that the tea itself has changed.
Short infusions deserve closer attention to the full sequence. Filling a small vessel slowly and then taking several seconds to drain it can make up much of the total contact time. A recipe that says ten seconds may count only the pause after filling, or may include more of the process. Read the explanation, and keep your pouring pattern steady when comparing attempts.
Use the number as a reference
Your preferred result may arrive before or after the packet's suggested time, depending on dose, water and vessel. Use the recommendation for the first brew, taste carefully, then make a deliberate adjustment next time. Do not shorten the preparation of ordinary herbal or fruit infusions below the product's safety directions. For those drinks, the required boiling water and minimum steep are not simply flavour preferences that a timer experiment should override.