None! The diy r134a kits are more for a 'little top off' if you're refrigerant is low. The best thing to do is to get a proper evacuation and refill, which as long as there are no diagnositics involved (i.e. your system works completely fine with no leaks, and is just low) is fairly inexpensive. A shop with a proper a/c machine can evacuate and recycle/reuse the refrigerant they recover from the vehicle, so there is usually a minor cost for refrigerant, typically just the difference of how much was evacuated and the difference of how much it actually took to fill to proper level/pressure.
All of the diy charge kits that I'm aware of, actually contaminate the r134a, and if you did in the future get a full evacuation and recharge, any refrigerant recovered from the vehicle would be wasted and would require all new refrigerant, not to mention can contaminate all refrigerant stored in the a/c machine if you don't make the technician aware you have used a diy kit in your system.
Typical cost is around 100 bucks for an evac and recharge on a proper working a/c system.