I am lucky that I can open a document of an article or chapter at any time and do five or ten minutes of work on it. Many writers tell themselves they need at least an hour, or at least an entire day, or five hours, or a week without interruption, in order to get work done. Some writers say they need the semester to be over and have the entire summer in front of them before they are able to work. Or they know that significant work will only happen during a sabbatical. The problem, then, is that they don't bother with shorter periods of time at all. Because they have in their minds a minimum period, they will never even see if they can get lucky too.
If you have a minimum like this in your mind, then re-examine it. If you need three hours, see if one hour is enough. If you need a month, try a week. If you need one hour, try twenty minutes. Just try it once as an experiment and see what happens. You may need to acquire the skill of being able to work for a shorter time, but it is a skill worth acquiring.