It's not a discount - but a hedge against price inflation. The cost of the cruise gets more expensive (typically) as time transpires. Early booking locks your price in at the current rates. This is especially important if you want to cruise during the more popular (read expensive) times of the year.
You will read old threads (and others will testify) that they checked the price just before sailing and recount significant price increases as the sail dates approach.
I don't think there is a percentage, or anything like that you can bank on.