Language is a skill. Learning English is similar to learning to play a musical instrument, or drive a car, or play a sport like football. Developing any skill takes time and lots of effective practice. To become really good at it, the skill must become automatic. This means it can be done by the unconscious mind, i.e., without thinking. For language skill, this is called fluency.
Getting to the point of fluency in English requires focused practice over an extended period of time. It is difficult, however, to maintain good concentration if study sessions are too long. DynEd recommends that students study frequently, but in relatively short sessions of less than one hour at a time.
Learning a new skill involves changes in the brain -- the growing and strengthening of new synapses and nerve connections. These are organic structures that take time to develop. Just like building muscles in the gym, there is a limit to what can be done in a day. Muscles need periods of rest to re-build after being worked hard. Brains also need periods away from studying so that there is enough time for the newly reinforced connections to grow. It is much more effective, therefore, to study in shorter sessions distributed throughout the week (3-5 times per week) than in long, intensive sessions on just one or two days. When study is distributed, learning English actually takes less total time, and much less is forgotten.