The workshop manuals even say that the A/C system needs periodic regassing to keep it in a working condition,so its pretty much normal if it hasnt been done for a few years..A/C systems are never 100% sealed,and have very slow leaks,some will go for 5 years or more without the need of a regas,some will last just 2 years before they need a top up.Also a lot of people dont run the A/C in the winter time,the A/C should be run for 15 minutes every week to keep the systems seals etc lubricated so they dont dry out and leak.Theres a high/low pressure switch that stops the compressor from cutting in when the system is low on gas.Also ask the person who does the regas about getting the dye put in the system.This dye will show up if there are any system leaks.