GO to this website for some helpful info..it will suggest that hot water heaters should last between 10 to 15 years....of ccourse many factors affect the real life...some of them may even make it to 20 years.... http://www.doityourself.com/plumbing/waterheat.htm
Usually the hot water heater will develop a leak into your catch pan...that is a sure sign that it is time for a new one....other signs to look for are how well it is performing...most of them will develop significant mineral deposits at the bottom over many years of use...and you can hear the deposits boiling off the bottom --therefore the water heater can get noisy while in the heat mode....Good luck and hope this was helpful....
How old is yours and why are you starting to question if it is time to change it out?