Towels are not safe, if chewed and swallowed they can get tangled in the gut. Fleece is safe because it doesn't unravel into strings like other fabric, so you have time to take the fleece away before too much is swallowed to cause an issue. Not being easily shreddable can make chewing it less fun for the chins, so most don't bother to chew it. However if the chin does chew the fleece it's not safe, it's not digestible and can cause a blockage if too much is swallowed. Depending on the bar spacing on the floor make sure toes and legs can't get caught. You may need to cover the floor in a solid surface or get a pan that can fit the bottom instead.
As to your actual question, it depends on the chin and if it's pee trained to go in a box or not. If not pee trained, fleece needs to be changed generally a couple times a week, some change it daily. It really depends on your chin and how tolerant you are to smell, some chins pee more then others and some chins pee just smells more then others. It also depends on what you are using as your inner layer to absorb the pee, I just use 3 layers of fleece, but other use a towel in the middle. I personally don't use a towel just in case the chins do decide to chew it, it's just more fleece inside. Wash it with vinegar as your cleaner, don't use detergent or fabric softener. Detergent can have harmful chemicals and some contain some fabric softener, and fabric softener reduces absorbancy.