How to use the online timer
Thirty minutes suits a daily guitar session, and for a beginner the fingertips will decide the length before anything else does. Soreness in the first few weeks is unavoidable while the skin toughens up, and pushing through it produces blisters rather than progress; several shorter sessions across the day are kinder and work just as well until the calluses arrive.
Chord changes are what most beginners actually need, rather than more chords. Picking two shapes and moving between them repeatedly, slowly enough to land every finger at once, does more in ten minutes than an hour of strumming through songs where the change is fudged each time. Barre chords are a separate problem, mostly of thumb position and wrist angle rather than of strength, and they respond to short, frequent attempts rather than long, painful ones.
Tune it before every session, since a guitar drifts overnight and practising against a flat B string trains the ear towards the wrong thing. A metronome exposes the timing problems that vanish when practising alone, particularly the habit of speeding up through easy bars and slowing down for hard ones. Split the half hour between technique and actually playing music, or the practice becomes a chore and stops happening.
Common questions
My fingertips hurt too much to practise. What now?
That is normal in the first few weeks while the skin toughens. Shorter sessions spread through the day are kinder than one long one, and pushing through soreness produces blisters rather than progress.
Why can I not change chords fast enough?
Because speed comes from landing the fingers together, not from moving faster. Practise two shapes back and forth slowly enough to place every finger at once, and the speed follows.