Princess recently cancelled an excursion that I had booked (they didn't advise me but that's another story) and the money was refunded back to my card in about 5 business days.
If it was refunded as OBC I don't think you'll see that on your folio until you are on board. That happened to me a few years ago. A recent cruise I was on also had a cancelled excursion (again not advised but for another day) which I found out about when I boarded and it was refunded as OBC. On the positive side that OBC is refundable so you'll get it back if you don't use it.
When you board I would go to the Excursion Desk and confirm the excursion you don't want to do shows your cancellation and confirm the refund has been made. That's what I did with my recent cancellation. It is much better to sort this out with a face to face as they can see the looks of impatience etc come across your face. As many have said, phone calls are not worth the paper they are written on.