As tat2_to_dive states, it depends, but there are minimums.
Obviously after each dive the equipment should be washed. The regulator should be rinsed well making sure that you rinse the first and second stages, hoses, consoles ect. Make sure that you have the dust cap securely on the first stage before rinsing, and make sure you don't press the purge valves while rinsing under water. Inspect the hoses to make sure you didn't abrade or cut them on the dive. The tank should be rinsed, and if there are boots on the tanks, you should remove the boot periodically to check for corrosion. The BCD should be rinsed well. I like to use a dump tank, then with running water, I rub down the entire surface with my hand. I pay special attension to any metal surface or buckle, making sure they are well rinsed. Then run some water into the the bladder through the oral inflater, fill with air and shake. Turn the BCD upside down so the hose is at the lowest point, and open the deflate valve and let the air and water out. Do this rinse cycle a few times to make sure all the salt is rinsed out (I look for the taste of salt). Wetsuits/gloves/boots also should be rinsed inside and out. I again use the dump tank and hand rinse. You can also use shampoo to clean the suits. Turn it inside out and hang and dry, and when dry, turn it rightside out and hang back up. All the rest of your equipment should be washed similarly. Anything that can be taken apart I take apart periodically in order to rinse out any salt and clean off any rust. Obviously the tank has to be VIP once a year and Hydro'd every 5. But if the tank is used an awful lot, or it gets banged around, you should have it checked more often.
As far as servicing, at a minimum, the regulator set and BCD should be serviced at least once a year. The regulator(s) should be broken down, serviced and tested by and authorized dealer, and the BCD should be also checked, especially if it's your alternate air source. The Dive shop should break down the whole BCD, remove the bladder and wash and sanitize it.
This should be enough for average use. But, if you do a lot of diving, or if the diving you do is in very silty or otherwise dirty environments you may want to do the servicing more often. Your dive shop should be able to give you suggestions on how often to service. Take care of your equipment and it will last a long time. I've had some of my equipment last for over 20 years.
Hope this helps.
Bill
