I don't know the exact interval they recommend, but it's a LOT longer than the Burgman 650. That was the deciding factor that put me on a Silverwing. I ride long distances and didn't want to be adjusting the valves every week.
Don't quote me on this......but most non-hydraulic systems tend to loosen with time, rather than tighten, so you COULD just listen for clicking. It won't hurt anything. Too tight, on the other hand, can burn valves. Somebody with more Silverwing engine experience than I could probably answer that riddle. But I'd guess you're OK to just ride until it starts to tick. That's actually the recommended procedure for a new Honda Civic engine: As long as it's not ticking, it's fine. And they seem to believe it'll remain "fine" for a very long time. There is no specific service interval.