It will take you any where from 5 to 20 mins to get back to the ship.
But you have to factor in a few more things...
You will need time to get back to the pier
You will need time for the tender
You will need time once you are back on the ship to " Just drop this off at my room"
You need time to get the to the windjammer
You need time to get food
You need time to find a seat and eat
You need time to " everyone use the bathroom before we go"
You need time to tender back
You need time to find a cab
You need time to get where you are going.
Overall most likely 1½ to 2 hours
You are better off going to the beach and having lunch there
Lunch for 1 is about 15 bucks or waste 2 hours trying to save 15 bucks
The time you saving and having lunch on the beach is worth it.
Don't forget that the time to be back is ship time NOT Grand Caymen time, which is 1 hour earlier.
Hope this helps