A high-quality hot water heater should last for many years before it needs repair or replacement, but even the best model will eventually break down and need servicing by a plumber. Obviously a lack of hot water is one reason to call a plumber for repairs of your heater, but note a few other times when you'll need to repair or replace that heater, and then discuss these issues with your plumber as needed.
Pilot light keeps going out
First, note if your hot water heater is located near a draft; for example, if the heater is in the basement, are the windows properly sealed, or is an open staircase allowing in a draft? This might be causing the pilot light to go out.
However, if this isn't the cause of the light always going out, you want a plumber to repair this as soon as possible. The pipe supplying the gas for the flame may be dirty and need cleaning, or there may be damage to the pipes themselves so that the light isn't getting enough fuel to stay lit. In any case, a plumber can note the problem and make repairs or replace those parts as necessary.
Water is rusty
In some cases, your home's water may be carrying in sediment from outside; this rust might be from exterior plumbing pipes that have corroded or from the well on your property, if this is your water source. However, rust can form inside a hot water heater and then mix with the water as it's sent through the pipes. This is not only unhealthy, but rust can easily eat through the body of the heater and then water may start leaking out. This can lead to a flood and other water damage in the home. Even if you only notice a rusty smell coming from your home's hot water, have the tank inspected for needed repairs.
If you hear a banging sound from behind your home's walls when you turn on the taps, this often means the pipes are coming loose from their connectors and are banging around. However, if you hear rumbling, gurgling, or any banging sound from the water heater itself, it may have sediment build-up inside the tank or too much water pressure, or loose pipes and connectors are coming into or out of the tank. These can all be very dangerous and damaging to the tank and can be a risk of a flood. When you hear any noise coming from the tank, have it checked by a hot water service as soon as possible.