Water heaters can last 20 years and sometimes longer.
Most newer water heaters only last around 10 to 15 years and if you get more than 10 years out of a newer water heater then you're doing good.
Some older water heaters can work for several decades.
I have an electric water heater that is 45 years old as of now and it still is working good.
I'm planning to replace it soon due to not trusting it much longer.
However it may last longer than that.
Ideally though you should plan on replacing your water heater within 10 years just to be safe.
Even if it still is working and not leaking it could rupture at anytime and it's better to replace it before it causes more costly damage to your house.
You should also put aside some money to take care of future repair costs and replacement costs of water heaters and other appliances in the home.
Also be sure to drain the water heater yearly with a hose and flush it out to remove any sediment build up which can extend the lifespan of the water heater.