When to replace my modem before it fails?

Hello,
I work from home full-time which requires the use of a wifi modem router combo. I purchased a used Arris N300 Surfboard Docsis 3.0 Cable Modem & Wi-Fi Router - Black (SBG6580) a year and a half ago. No problems, it works great. However, are there any telltale signs to alert me to purchase a new combo as I'm concerned my current modem will fail while at work, with no access to the internet. I'm considering purchasing another combo, but having it sit for a year or more if the current modem continues to work is something I would like to avoid.

Also, my modem, pc, etc are on a power strip which I turn off at night, from 10pm to 5 or 6 am, so the modem is not running 24/7.
Thank You

I didn't find the right solution from the Internet.
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