If done correctly and the fan unit isnt damaged, it should last awhile. Although if you have to revist the unit and redo the process, the IC diamond is going to make getting the heat sink/fan off the chip/board a real pain in the ass to be honest. I've had to do that once already and thought i was going to warp/break the board if i applied more force in the process.
 
I don't know if you are going to get a definative answer on this to be honest, but i'd treat the system carefully like an elderly person to try and get the most time out of it to avoid performing that surgery too often.

My first reflow on my launch ps3 lasted around 7 months before it started ylod'ing again. I then reflowed again (using flux that time) and it lasted about 2 months... As for how long your friend's will last, well, it's really impossible to tell given you can really tell how well the reflow took. You could create more bad places than repairing good places and that will just make it fail sooner.
 
  I found a guy on ebay that reballs ps3's with LEADED solder balls for 60 dollars +15 shipping. You really can't beat that kind of service and you'll never have another problem with ylod because of bad solder joints again! (My ps3 is currently on the way to him :) )  If you have this service performed on a virgin ps3 (no reflow attemps) you have close to a 100% success rate, however, every reflow attempt lowers your chances as it weakens the traces in the pcb and they can possibly be pulled up during a reball when the solder is removed from the board :( The reason the leaded solder will prevent this from happening is because leaded solder has memory and as such, when it's heated to melting point and cooled back down, the solder will create the same joint as before. Lead free solder, when heated and cooled, will start to form small fractures within itself, eventually causing a bad solder joint in one of the many joints on the underside of the chip :(
 
If you want a link to his service, send me a pm and I'll message it to you :) I'd highly recommend that be done.  It's sad that leaded solder can't be used in a ps3 because there's a possibility a kid might take it apart, and stick the ps3 in his mouth... lol

I have done this to my PS3 V1.00.  First time it worked for about 3 months before going out.  Second time lasted only a week.  So instead of a third time, I bought a 3000 edition PS3 slim to replace it.  Build quality difference is worlds apart, I like the fat PS3 better.

The sad thing is you shouldnt have to fix them. This generation's consoles are some of the poorest designs i've ever seen. I had to fix my Xbox 360 once already and once you crack one of them open you realize how crappy they really are. They really thought 2 60mm fans were enough???

Check out these videos on a "proper" repair of a broken PS3.
 
Repairs costs can be as low as 50 dollars and as high as 150 dollars if you send it in to Sony for repair.
  


 
Here is a site that does reballing. I think their price is $80.
http://ps3repairman.com/ps3-repair-services/ps3-reballing-service