How often do air conditioning units need to be charged with freon?

We have been having terrible problems with our central air conditioning unit. Our house is about 8 years old and we have a zone system (separate heating and cooling thermostats for upstairs and downstairs). For the past 2 years the unit is not cooling the house properly (heating works fine). We had the dampers replaced 2 years ago and I know the dampers are fine. We have had several repair technicians look at the unit. They always recharge the unit with more Freon (3 times in 1 summer) after claiming to fix a leak. This past weekend again a repair person claimed that we had an internal leak in the unit. He said that he had plugged the leak and then he charged the unit with Freon. Not only did this not work (our house continued to get hotter from 73F to 85F while AC was running all day) the pipe coming from the unit outside our house was all frozen over with ice (all the way to where the insulation started).
Any suggestions or help will be appreciated. Thank you.

A system only needs to be charged once, when it is installed. If it gets a leak, the leak must be repaired, and verified that it is repaired with a vacuum test. It is a laborious process, and some homeowners would rather just juice up the machine each year than spend the funds to get it repaired.

Some units have leaks that can never be identified. There are lots of fittings, small bends in the coils, and hundreds of feet of tubing that need to be traced with an accurate tool. Many Tech's don't have access to this tool, and rely on soap bubbles to identify leaks.

Contact the manufacturer of your unit, and have them recommend a competent company in your area. If it is the company that you've been using, explain that they have not fixed the unit. For very hard to find leaks, there is a product available that can be added to the refrigerant similar to "Stop Leak" for your auto radiator. This product is rather spendy, about the same price as five pounds of refrigerant, but if you are charging your unit each season, it would be worth the investment.

Contact the company that "repaired" the leak last time, and ask how the technician verified that the leak was repaired, and how he verified that the charge was correct.

I do not think that Freon is ever used.It's Ammonia.Time for a new unit.

Freon theoretically lasts forever, there should never be a need to replace or recharge the unit. You still have a leak, otherwise you'd not be getting the ice forming. My fridge did the same thing (ice and no cooling), and an HVAC tech friend of mine said that it's because the freon had leaked out. I asked the same question you did, and he said that freon should not 'wear out". Get that leak fixed (again) and recharge the unit. If the leak is sealed, you will not have the problem.

mine was 2-4 times during the heated weather. arizona.

Your A/C unit is a SEALED system. That means that if the system is leaking then there is a need to get a COMPETENT A/C technician to troubleshoot the problem, find where the leak originates and repair it.
Your main problem is that you are using people who do not know what they are really doing and I am sorry to say that these cowboys drag the reputation of the industry down.
At some point you will have to decide whether to keep using the people who have returned and not repaired your machine ...and... finding a really good technician who can genuinely fix your machine. All I can say is good luck because these guys are as rare as hens teeth and if you find one you want to keep their details and pass them on to close family and friends.
They might charge more for their time but they are worth every cent because they will get the job done right first time.
Sorry if I sound a little harsh but I have no time for so called A/C technicians who cannot do their job properly - good luck!