avg 50k hours? not a standard sleeve bearing fan, maybe a very good hybred sleeve fan if that, like for example the sintered oil impregnated sleeves found in Nidec and Papst. Avg life span in a standard sleeve bearing like this one found in Zalman (haven't found out who there manufacture is) in likeable testing environment would be 15k-10k hours. Of course in real world drop that significatly throw in dust, heat, drying out, odd mounting positions, abuse (like us holding the hub down to stop a fan), inbalancing, uneven wear, manfucture defect. Im sure you've experanced it you got a fan with your case maybe a Sunon sleeve fan MTBF 20k it barely last 6 months before it starts to whine. Only look at the MTBF as a general benchmark, don't expect near that lifespan (or maybe even half specified in some cases).
Generic Brand Sleeve: ~8k
Nidec Sleeve: ~25k
YS Tech: ~20k
Sanyo Denki: 28k
Sunon Sleeve: ~20k
Delta Heavy Duty Sleeve: ~33k
Nidec NBR: ~55k <-hybred sleeve
Papst Sintec :~45k <-hybred sleeve
Panaflo Hydro: ~33k <-hybred sleeve
GiZzO's Fan Database Page