When you need to use eye drops frequently, the absence of the perservative makes a difference because many people actually get irritation from exposure to the preservative. For this reason, for individuals who need to use artificial tears frequently, the preservative free products seem to be a very good idea. With regard the the frequency, it is essential to understand that the drop only sticks around 30-40 minutes and then it is gone. Whenever help it was doing is also gone. For this reason, if you are having a bad day, you might find it beneficial to use the drop every hour during the day. If you are going to bed and the eyes are bothering you, consider using a bland ophthalmic ointment like Refresh PM at bedtime. You only need a few mm of ointment. Instill it by pulling the lower eyelid down. It will melt and coat the eye so you don't need to do this which can risk scratching the cornea. If you find yourself frequently having bad days, it can be very helpful to also see a cornea specialist. They can work with you no the use of antibiotics to improve the blepharitis, plugs to help trap moisture and other measures that can help eye comfort.