I usually schedule the message to go automatically at midnight.
You'll get a reply by the next business day at the latest saying they've "calendared" your requests and indicating if there are any they think might be a problem.
For cabanas, I've always given a first and second choice specific number and then a range as a third choice. For example, on this cruise I said first choice was #5, second choice #4, and if both of those were taken, any cabana #1-9. (We like being close yo bathrooms and food.)
They are very nice and will answer other questions too about procedures, refreshments in the lounge, etc.