Quality time is essential in any romantic relationship. While there are truth in the saying, absence makes the heart grows fonder but too much time away can bring a huge distance in a relationship.
Determine what is keeping him from you. Is it work-related? Friends? Does he live in another town? Or is it his choice to be stay unavailable throughout most of the relationship and see you whenever he feels like it? If it is the last one, maybe it is for the best to end the relationship, as you are not his first priority and to find someone that values spending time with their girlfriend.
Manage your time together. Work hours can be exhausting and it is normal for anyone to feel like hitting the sack after a long workday. If he is tired during most of his off time, offer to give him a visit and give him a good backrub to relax him and mention if there is any room left in his schedule that includes you. Or go to his workplace during his lunch hours and surprise him with his favorite snack. Most likely, your boyfriend will try to make an effort to see you more. Even if it is two or three hours a day, it is much better than going four straight days without him.
Make him want to see you often. The biggest problem for some females not seeing their man a lot, he is spending too much of his time hanging out with friends. While it is healthy to have some 'me' time and divide time with others but if he is doing everything with them (going to the movies, hanging out at his house, and so on) and hardly does it with you, try to distract him and show him things his friends can't do. Send a sexy text message or wear a nice set of lingerie and that will definitely put a smile on his face and he will minimize hanging out with his boys and come running to you.
Pick a day out of the week when it only involves the two of you and no one else. That day should have no distractions where you can spend time with your boyfriend all day long doing anything you both like. If this plan proves to be successful, gradually it will become more days like this.