This is a question I am often asked. Ideally, of course, it is best to set aside 20 minutes daily. However, it is often difficult to get into a routine like this, so the most important thing is to get into the habit of trying to play every day, even if just for a few minutes. It is best to keep the guitar easily available, ideally on a stand, then, rather than immediately ploughing through the latest challenging piece, simply play your favourite songs and pieces (after a suitable warm-up, of course!) After this, you could then perhaps practice your latest piece or exercises in a more focussed way.