I bet their mechanics probably can do it under two hours clocked from pulling in the vehicle to parking it back in the customer lot. I would not be surprised if their guy could do two at a time on adjacent bays. I read somewhere that if you hand them the car in the morning, you get it back before lunch time. They also take you to local mall if you want to spend few hours there.
I got a coupon from my local Honda dealer for $390 special too. It is possible that some of the dealers have started replicating Hamilton's method.
Back to the original topic; I waited lot longer that 7 years / 105K miles and the timing belt which came out was pristine. So the odds of OP's belt or tensioner giving up any time soon is miniscule. Although for $500, it is no brainer to get it done right. You should plan on changing the belt twice and keeping the vehicle for 15+ years.