My experience is that its almost immediate - certainly within a week or two.
Biggest benefits are 1) the EQMs and 2) the AC membership. I use the AC most every time I fly so that's worth it to me. And as others have mentioned, you can get things done at the AC (in terms of IRROPS and rebookings) that regular agents cant or wont do.
I also hear the travel protection when using the Exec card are pretty good when things really go bad (over night busted connections, long delays).