This is a hard question to answer since it varies so much from person to person. It depends on your style of grinding and clenching. We like to say it's similar to running shoes. The average runner might have shoes for years while an Olympic athlete might get shoes every month.
The Hard flexible seems to last the longest out of all the guards we sell. We have customers who order new ones every 6 months. We also have customers who have been using the same guards for 5+ years. It depends on the severity of your grinding/clenching. With proper care and cleanings, the average night guard will last 1-2 years.