I've had DSL Modems last for 2 years and then I had a DSL internet modem last for 10 years without a problem.
I would say that on average you should be prepared to replace your DSL modem within 5 years of purchasing it.
However it may last longer than 5 years but 5 years is a pretty long time for a DSL modem to last.
Power surges can damage a modem sooner than it's expected life expectancy so be sure to put the DSL modem on a surge protector to protect it from power surges.