I have 8 of them lol. I change my filter pads once a month, or every other month if I forget lol. It depends of what size tank you have them in and how dirty your fish is. I have one boy in a 3 gallon who is a very.. VERY dirty fish. Back when I had him in a vase I had to change his water every day, when everyone else was okay with every 3 or 4 days.
Anyway, I love the filter and I always tell everyone to buy this one to replace the ones they already have, like in the Aqueon tanks and stuff. It's safe to do it every month, and when you change the pad you should probably do a partial water change if any gunk comes out from the filter when you pull the pad out.