Priority Boarding doesn’t change when you can play the exiled card. For example, if you exile a sorcery card, you can cast it only during your main phase when the stack is empty. If you exile a land card, you can play it only during your main phase and only if you have an available land play remaining.

Once you choose to reveal the top card of your library during the resolution of Priority Boarding’s ability, that ability won’t trigger again that turn, no matter how many more dice you roll. This is true even if the card you reveal has a mana value too great to be exiled or you reveal a card you could exile but choose not to.

If you roll several dice at the same time, that will trigger the ability once for each roll. You can choose which result you want to reveal a card for.