That's hard to determine. Roof inspections are usually done several times a year during cleaning. That's when you'll decide whether to seal. Watch the sealing videos available online and you will become familiar with what to look for. Also, check the manufacturer's (Alpha Systems?) website. They have the information you need without a lot of hype. You will become familiar with the products and procedures quickly. I seal probably more often than I need to, but if an area doesn't look exactly right, it gets sealed. It doesn't have to be a neat professional application. If it creates a seal that keeps water from getting into your TT - that's good enough.
Your dealer didn't need to scare you, but they were right to inform you to keep your roof sealed. Sealants are inexpensive and easy to apply. A small leak can create expensive damage. I didn't want to say this up front because it sounds like a wise-crack, but the answer to your question is: As often as it takes to keep the roof from leaking.