On Silhouette TA last fall, we found ourselves with a lot of leftover OBC after I had done my worst at the spa. We haven't traditionally gambled. Four days before the end of the cruise, DH and I each put $100 from room charge on slot machines and played a few spins. We got lucky and ended up $15-20 ahead, so we decided to cash out. That was fine.
Next day, we went back and found we were locked out. However, we had a few promo dollars and played them. Turns out I liked the slots (uh-oh!). A friendly fellow passenger said we would have to wait three days.
We checked every day, and sure enough on the last day of the cruise, we were able to transfer money from our room to the machines. So we each put on half of the OBC we wouldn't use in tips, and went to town. This time we played longer because it was fun. Still ended up a small bit ahead. So we cashed out our OBC and also had some fun.
I don't know what percent of the amount transferred you have to risk to avoid being locked out. But on our next cruise we will just play as we want during the cruise, and play to cash out on the last day.