Was it with Andrew Bowe? He's an awesome teacher, along with awesome farrier & horseman. Highly recommend his workshops.
I first learned to trim my own horses long before there were any 'short courses' like that. Went to the knackery to get hooves to practice on. Never imagined back then I'd do it for a job. I've taught many how to trim.
How often? I do my horses on average about 3-4 weekly. *Keeping* them in shape with a light rasping rather than wait for them to get overlong before trimming. Of course, life gets in the way & they might go substantially longer, but generally... If there are any issues - seedytoe, cracks, hooves are a 'work in progress' etc, best not let them get worse between trims & may even be best to do them more than 3 weekly, to fix the probs.
Some horses, depending on health, lifestyle, abrasive terrain etc, can go longer between trims. Most horses, depending, can go 6 weekly without significant problems but it is generally best for them to be trimmed more frequently so overgrowth & any iimbalance dont get out of hand.
I suggest, if you're just learning to do the job yourself you keep a good farrier on as well. While you're getting your skill up, start with them still coming 6 weekly, if only to check your work, then gradually stretch out the visits as you get good at it. As there are many little things about balance etc, I'd still advise having a farrier check your work every few trims for some time. Then just periodically thereafter.
You can learn a lot at a one day workshop & 'maintenance trimming' is not rocket science. But it's also important to appreciate your limitations. It takes a LOT more to understand all the factors well, that go into good trimming, hoofcare & especially managing problems. I'd absolutely suggest you call a pro for any issues you might encounter too.