How do you know when it is time for a haircut?

I got my hair layered four months ago and now it looks like the layers are coming out of my hair and my hair is also starting to look a little dull. I don't know if it is because I put alot of heat on it during my last school year or my hair being puffy all together makes it like that. But my question to you is how do you know when it is time for a haircut?How do you know when it is time for a haircut?
i think its time for a haircut when u dont like it at the length it curently is,or u see dead ends.maybe a trim will work,and for the dullness i use hearbel essance


and i love the shine in my hair :DHow do you know when it is time for a haircut?
four months, its about time for a cut, especially if you have layers... for me, i know its time for a cut when my hair just looks puffed out and dull and starts to feel dry. edit : if you go for a trim just tell them you want to keep the layers. they will not give you a blunt cut unless you ask for one.
Well personally I know it's time for a haircut when there's an awful lot of split ends :) and that happens often as I also straighten my hair regularly
I guess since your hair looks dull.