Sorry to be so strong about this, but the concept of "topping up" is wrong.

The fact is that if your air conditioner needs a "topping up" once a year (or more), it is leaking.

If your car has a leak, then bring it to a competent A/C shop and have the leak repaired.

An A/C leak is not a benign thing--you are leaking refrigerant into the environment. And, although R134a is not as bad as R12 was, it is still a nasty chemical for the environment.

For those who don't know, there are two major areas of damage which result from the release of refrigerants. The first is ozone depletion. The second is global warming. Refrigerant R12, because it contained a chlorine atom, was bad on both counts. Refrigerant R134a does not contain a chlorine atom, so it is not an ozone depleter. But it is a string contributor to global warning!

Do the right thing and fix the problem, don't just put Bandaids on it.