Regardless of plan/payment, the standard non-alcoholic drinks (tea, sodas, lemonade, coffee) are always refilled free.
Some places (like WCC) offer 'premium' non-alcoholic beverages with free refills too - like the bottomless milkshakes. This is far less common.
At breakfast buffets, juice and milk are refilled free; often at non-buffet breakfasts, or anywhere for lunch and dinner, they are as well, but some places do not do this for milk or juice. For these, I would ask if you are unsure.