Usually they stay the same for 3-4 years with most manufacturers, but it really depends. Usually there are small changes from year to year, but the "body style" will stay the same for a period of time, just like cars do. There is no database of hot tub changes from year to year, a knowledgeable dealer can usually tell you the changes. I'm pretty sure the 3 series Jacuzzi had some significant changes 1-2 years ago.