How long should it take for an Attwood 6 gal propane water heater to get up to temp? I have heard people say to turn it on 10 min before you need it but mine takes about half an hour to get hot.
Take a look at the flame and see if it is blue or has some yellow, which would indicate a cooler flame. You could also have a dirty heating chamber which is keeping it from transferring the heat from the flame to the water.