I built a loop on the cheap using 2nd hand parts. I didn't put a drain port or anything as I couldn't be bothered. I didn't drain if for a couple of years or so, but then did a gpu upgrade so I thought, great! I can actually drain and flush it now.
Popped out the gpu and unplugged the mini res / pump, but hardly any water came out as it was all still in the rads.... tried to flush some water through but couldn't make it work.
That was a couple of years ago, so the bulk of the fluid has probably been in the loop for like 4 years. Fortunately, nothing bad has happened, at least nothing that I can see anyway. I'm just running deionised water + a drop of some kind of adative though.