there is no general rule as to "when it's too late". Most mass-market cruise ships sailing from U.S. ports sail at 100% occupancy or more (100% occupancy is the number of lower beds, while upper berths or other extra beds can result in higher occupancy rates).
Your travel agent should be able to see how well the cruise is booked and he or she might also be able to tell you how soon a cruise at that time usually sells out. But please remember that there can be no guarantee whatsoever that the ship will have occupancy left at a later point.
You could very well be lucky, but you might be facing a disappointment. Booking early avoids this. I know it's easy for me to say this, but sadly, that's how it is.