Remember that all routers are not the same. Just because you have backtrack and Alfa doesn't guarantee you will crack wep! I think even the aircarck-ng site says there is a little more art than science to cracking wep.  Some routers have some recent firmware upgrades that do a reasonable job in making you drop packets, etc... What you have to do is try different injection speeds, play with the kind of packets you are injecting, etc....  There are so many variables to consider. Play with the distance. Your first attempt should be very close to the router. Dont be rigid about your approach, if you do you will never be a true hacker. 

I think we are in desperate need to coin a new phrase for "think outside the box". Can we do that Don as a contest,LOL?   Think outside the box has been said so much to the point it seems to not register anymore.  Kind of like when your mom said "eat you vegetables". 

There is no set amount because of variables you will encounter. I have cracked 128 bit with less than 200,000 and sometimes it has taken 2 or 3 times that amount. 

From my expierence, to crack WEP-104 need collect  40 000-90 000 IVS, for WEP-40  ~10 000-20 000 IVs.