Gas hot water heater issue?

[11Andy]

I have a three-year-old 50 gallon Rheem gas hot water heater. Until recently it worked flawlessly and provided hot water to my shower within about 5 to 10 seconds. lately, however,it has taken more than 40 seconds to get the hot water flowing. I haven't adjusted the thermostat or touched the unit at all. What could be causing this change...do I need to do some sort of maintenance to the unit? Thx!