Modern AIO's are designed to last for years with no maintenance, some like BeQuiet come with a top up function, obviously cheaper may not last as long as premium, your looking around three to seven years apparently, but even an air cooler should be serviced every three to five years, as thermal pastes need replacing regardless of cooling choice, and fans/heatsinks need cleaned.
The paste on an old MiniITX here for instance had what is known as phase separation/silicon bleeding. Basically the thermal paste had broken down, the CPU looked like it was wet with an oily looking substance and had clumps of grey dotted around.
The last AIO I had fail was a CoolerMaster Nepton 240M that lasted 10 years in the front of an NZXT S340 before the pump started making noises, its a non serviceable item, so I think 10 years was actually really good, so that was put in the scrap and replaced with a BeQuiet Silent loop 240.
10 years for an AIO costing £80 is pretty good, 5 years would be good. But if you want longevity and an easy life, an air cooler wins every time but just don't do what I do and leave it for over 5 years, remove clean and repaste on occasion.