focus period depends on you as an individual and how long you can sustain focus without your mind getting tired or bored and wandering off. i find that between 30-50 minutes of focused time is best. you can start off with a 30 minute focused session and then gradually work your way up to a full 50 minutes of focused work.
take a short break between each session of around 5-10 minutes. try to do something physical and get away from your desk during your break. you could make yourself a cup of coffee, get a snack, go for a brisk walk or do some stretches.
after two to three sessions like 