Thats a how long is a piece of string question. Most of the problems with underwater detectors are down to user error added to bad design. You don't have to have batteries in a control box next to the circuit board as many of the more expensive have. Most Chinese, yes I know pretty bad, water machines have the battery pack in a seperate compartment so the important seal is never disturbed. Its just so easy to slightly twist or crush a seal as the detector case is closed, even a grain of sand could lead to a leak.
With "land" machines I've old ones that have never needed any work. Modern detectors seem less reliable but then there's so much more to go wrong. Now we have the chance of G.P.S. failure for the first time ! One thing I have noticed is that many of the modern slimline coil designs seem to go off tune after a few years.
Ignoring reliability, the best is what suits you and your sites best. If you sweep fast you had better get a detector that allows a faster sweep. A heavy/out of balance machine might go an inch or so deeper but if your going to stop detecting after a couple of hours due to arm or back problems then you will have more finds using a less deep detector that allows you to detect the full day out. Then there's the fact that whatever you spend some detectors suit some soil conditions far better than others. All I would suggest is that you up the odds of your detector working well by at least selecting one with a ground balance control. If it says automatic on the box it normally means fixed which is not good. Ground balance control also means that any size or type of coil is optimised to the detector each time you use it.