Hopefully you won't have to replace them very often. They tend to be very unreliable...though occasionally some will last several seasons.
It's one of the very few items that an extended warranty actually may make sense for. We had ours replaced last year by Frigidaire / Electrolux.
It's a pity Frigidaire discontinued their 70-pint dehumidifiers. I have 2 Frigidaire FFAD7033R1 units (one upstairs and one downstairs, that run 24/7 Spring through Fall to keep the indoor RH around 4x%).
I previously had just one and it failed about 2 years ago. I tried to find a shop that could repair it, and wound up calling Frigidaire CS for advice (in the meantime, while trying to find a place to repair it, I'd ordered a new one from Amazon).
The Frigidaire CS rep asked where I bought it (Amazon) and how long ago (I said about 4 years ago, but I wasn't sure). He told me they had 5 year manufacturer warranties and emailed me the warranty return form.
All I had to do was peel off the label with the serial number and stick it on the form, fill out the form and email them a photo of the filled form with the label stuck on it, along with a photo of the unit with the power cord cut off (didn't need to snail mail anything). They didn't ask for proof of purchase/receipts etc.
Whaddya know, the brand new unit I'd ordered from Amazon didn't work, called CS again. Same thing. A week or 2 later, 2 brand new units arrived on my doorstep direct from Frigidaire. I now run one upstairs and one downstairs (and I use the drain hose attachment so I don't have to empty the internal tanks). Keeps the indoor RH at about 38 - 42%.
P.S.
For folks who read my post about the SensorPush setup, I have a bunch of their sensors now, that monitor the refrigerator and freezer, the garage freezer, the living room and basement. They continuously log temp, humidity (dewpoint with one of the sensors). I can also move them around if I want to measure/log different rooms/things.