uhmmm......tough question.
Adam was asked: when do you know when the album is ready? He answered: when it's in the shops.
I would really like the next one to be out in late 2006, but with U2 you never can tell.
consider this: U2, first and foremost, are ARTISTS, so they release a record when they FEEL it's done and ready. then, the media machine starts to gear up, etc...
anyway, looking in retrospect, U2 are a very prolific band. in 24 yrs (80-04), they released 12 studio albums (including R&H and Pass, of course). that's a pretty good pace, if you consider all the tours, solo projects, movie soundtrax, charity work in between, etc....that makes an album every 2 yrs!!!!!
on one hand, I'd like to see them release records more often (ala BOWIE/n. Young), on the other hand I'm perfectly happy with their work pace.
I definitely take quality over quantity.
just look at REM: between 83 and 04 they released 13 records, but how many of them are really good?