I've had mine for about 10 years.... and bought it used, have no idea how old it really is (there's no build date code on it, could be as much as 20 yrs old since the lens was first intro'd in 1993). Still works great and it doesn't get any particular coddling or special care. AFAIK, it's always been used with the lens hood (came with it), which seems to be one of the "secrets" to the lens having a long, useful life.
The best info I have seen has been from Lensrentals.com. They buy large quantities of lenses, check them after each rental, and make repairs as needed. Their stuff also gets used hard... really hard. I bet it spends more time bouncing around in the back of a UPS truck than on a camera, taking photos! And, even then, I suspect users don't treat short term rental gear with the same care they do their own stuff. Lensrentals sell off their inventory and replace it every couple years, I believe.
Lensrentals.com publishes a repair report for gear that focuses on higher than average problems (average, if memory serves, is something like 14 or 15% a year, or about 1 in 6). The EF 50/1.4 is on their list and the last time I looked showed about 20% a year repair rate, mostly due to AF failure. So, based on that I'd say there is a 1 in 5 chance of failure, if you treat the lens rather poorly. You can greatly reduce those odds a lot with some reasonable and easy care.
Not only is it possible to damage the AF with a hard bump to the front barrel (Use the lens hood when shooting! Set the lens to infinity to fully retract the barrel when stored, which actually is a good idea with any lens.)... It also seems possible to damage with a hard bump on the focus ring (Reverse the lens hood when storing the lens... it covers and protects the focus ring.)
Finally, it's also been suggested that because the lens isn't "true" USM, it might be more susceptible to wear and tear of the AF mechanism if you override AF a lot with manual focus, without first turning off the AF at the switch. A USM lens is supposed to be FTM capable, but some feel the 50/1.4 is less so. Personally I very rarely have need to override AF manually, the way I use the lens, so that might be another thing that's contributed to the long life my copy has enjoyed.
I wouldn't worry very much. If the AF broke on mine, I'd have it repaired. Last time I heard, repairing it costs about $160 US. If my lens were dropped and broken beyond repair or stolen of something, I'd buy another 50/1.4 without concern... And I'd always get and use the matching hood, too.
And if you are too worried, you have alternatives. The Sigma 50/1.4 seems a sturdy option. Maybe too sturdy.... For a 50/1.4 it's massive and heavy! Who ever heard of a 50/1.4 needing a 77mm filter!? Hell, the Canon 50/1.2L only needs a 72mm and older manual focus 50/1.4s were somehow made using 49mm, 52mm and 55mm filters! Maybe Sigma is going for a size and weight record. Anyway, who knows if the Siggy is really going to prove to be more durable... It's a relative youngster that's only been sold in rather limited quantities, compared to the Canon. After 20 years, I bet hundreds of thousands of the Canon EF 50/1.4 are in circulation (so it's not surprising to see a fairly high incidence of "issues" online). That probably compares to tens of thousands of copies the Siggy (in all mounts offered) at most, that have been marketed since it's intro 5 or 6 yrs ago. We'll just have to check back in 15 years or so, to be sure how the two actually compare in terms of durability.
Hopefully some day Canon will design, build and offer a 50/1.4 "Mark II" that addresses some of the "issues". Sooner or later they probably will. I ain't holdin' my breath, though.
