Whenever! Saline sinus rinse is a safe, homeopathic way to clean the sinuses of mucus and allergens. This may be done up to several times a day as long as no side effects (pain, bleeding, etc.) occur. There are rare case reports of serious infections with using otc saline rinse, but the risk should be minimal with using sterile water (boiled or from pharmacy) and extremely clean neti pot or rinse bottle.
Daily. Once daily is ok.
1-2 times daily. Since saline (salt water) nasal irrigations are not a medicine, there really is no answer to this. Most do it once daily, but after sinus surgery or during a sinus infection rinsing 2-3 times a day is safe. Contact your ENT if you are not sure.