How thick or thin your hair is depends on heredity. Thick or thin hair can still grow long and healthy but its length will also depend on what genes you have. Some women have genes for really long hair and others can only grow hair to their shoulders. I've seen some women with thin hair that is down to their knees too. Most women can grow their hair to about the middle of their back before it starts to get wispy and thin looking.
How often you trim your hair is another choice. You don't have to trim you hair at all if it's healthy. The only purpose of trims is to keep the hair shorter and thicker looking and to get rid of split ends if you are prone to them. If you do not have split ends and want longer hair you do not have to trim your hair. Remember, salons are in business to keep you coming back and most will tell you that your hair needs a trim every four to six week to grow which isn't true. Hair is dead and the grow rate has nothing to do with the bottom ends.