Never. Just being honest.
I have relatively new, good condition lines on my reels that I primarily fish dries with and need to completely float. When a line starts to get older, dirty, cracked, beat up, whatever or otherwise, and the tip starts to sink a little, I retire it to my Warmwater reel or my reel I sometimes fish small streamers with where I don’t mind (and may even want) the line to sink a little. You’ve made yourself a free sink-tip at that point and extended the use of the line a good bit.
At $40 or so for a decent fly line, and using the above retirement process, it just usually isn’t worth my time to clean it. I have 4 fly reels, and maybe purchase one new fly line a year…probably more like every other year. With relatively frequent use, no cleaning it, and in the absence of acute damage to the line, it seems I usually get 2 or 3 years of use before needing to rotate a floating line out of the starting lineup.