When does it become the time to replace your modem router?

I'm currently having minor issues using the internet at home. I use a wireless modem router and sometimes I get disconnected suddenly from the internet. I can still resume by turning off the modem and turning it back on which will get the internet to connect again. It is getting a little worse now than it did before, and takes a long time for everything to get connected to the internet after turning the modem back on. Sometimes, I would have to try turn it back off and on a few times to get it working again.

I feel it's about time to replace the modem, but I'm not sure whether having it for about 3 years requires a replacement or not. So after how many years of using a modem should I replace it?