How often should you have a home air conditioning unit recharged?

Whether the leak is miniscule or not is not the issue. If your system is leaking refrigerant, you must have it repaired. Any technician that puts refrigerant in a leaking system and does not identify the leak is not worth keeping. Answer is, since it is a sealed system, if the unit is losing coolant there is a problem, there is a leak. However, as a practical matter and since the leak may be miniscule, having your system checked every couple of years is not a bad idea.