The answer that isn’t really an answer is; it depends on how long your hair is. To keep it simple follow this rule.
If your hair is short, you need a haircut once a month. If it’s medium length where you can actually gel your hair, then once every three months. Any long hair also needs to be cut every six months to a year.
If you follow this rule you will be alright. Now I got a question for you, are you ok with just being alright? I don’t want to be known as the guy who dresses alright. So what else do you need to do?
Between haircuts things will start to grow in different places depending on how hairy you are. For me, I have an M shape hairline and the back of my neck also gets really hairy. So every three to five days I pick up my cut throat blade (I recommend you get one) and I shave off the hair myself. Just because you got a haircut four days ago doesn’t mean you still look nice and neat now.
If you aren’t sure if you need to touch places up with a razor just ask a friend if your beard or your hairline looks messy. Staying on top of this five minute job will make people look at you like someone who just always looks great.