Probably about two to three years. He started after The Beautiful Game and said he hadn't had an idea when someone called him with the suggestion for TWIW
Yeah.....definetly not from the time he was working on Joseph.....he said himself that he needed an idea about 4 years ago or so. WIW is what came out.
Obviously Bso was referring to the fact that ALW constantly steals from himself...Thus, since Joseph was his first major musical, he's been writing WIW since then.