1995 XLT 3.0 5 speed, nearing 300,000 miles

I searched through the other threads but didn't find anything that seems to specifically address this. Sorry if I missed it! For a couple of years now the a/c has been leaking the refrigerant. I've taken it to my mechanic and he was also unable to find the leak using the UV dye. So, he's been kind enough to just recharge the system (free) whenever it stops blowing cold air. Not something I want to keep doing...

It takes a couple of weeks to a month before I'm back to the hot air. Considering the fairly slow rate, and fact we can't find it, where would I likely find the leak? The a/c system is 100% original so maybe it's time to replace things? I'm fine with that, and am about to start doing so if I can't figure this out.

Thanks!