It really depends, but you'd most likely incur a hefty wait. Why not call Disney Dining after landing (or if driving when you're close) and see what's available for that evening. You should be able to estimate what time you'll be arriving by then, and be able to lock in a time. I've waited at Boma 30 minutes WITH a PS, so I can't imagine what would happen without one. I've been able to walk into Jiko, but it was at 5:45. I had stopped by Jiko earlier in the day, and talked to the CM at the podium. I asked him if there was anything available for that evening, and he told me if I got there before 6:00, I'd be seated no problem. By the time we left, around 7:30, Jiko was packed and people with PS's were waiting in the bar area. If both Jiko and Boma are unavailable for the evening, Mara is wonderful and is a great place to eat!