Great question, I would first determine if it was only affecting your wireless devices by hardwiring a computer to your modem / router and check that machine when you start experiencing issues. You may also want to investigate the lights on your modem to see if the internet light is lit up sold.
A good rule of thumb is to replace your modem / router every two years. If you are using a cable modem, most of the time if you visit 192.168.100.1 in a web browser you can check for errors with the upstream/downstream channels. If you notice any of these it is time to replace your cable modem.
I would also run a speed test using speedtest.net or fast.com to check and see if you are getting the bandwidth you are paying for. If you are not getting the bandwidth you paid for then attempt power cycling your modem / router equipement, if you see that you get better speeds after doing so it may be time to replace your equipment.
Let us know how it goes.