If you run the aircon system every month, 30 minutes as the manual recommends you shouldn;t really lose any coolant, as the seals will be kept in good condition. You will have minimal coolant lose, but you shouldn;t need to charge it for at least 2 years, if run every month. Halfords will check the aircon gas level with a guage for free. You can also buy kits to regas and guages, which will work if your aircon still works to some extent. As said above they don't really work if your aircon is completely gone, although you can get aircon dye kits with glasses and leak sealer, etc.
The haynes manual goes through the regassing process, you just put a guage on and then regas through the same port.