Sure enough, 45 days later and a rescission package has been announced.
http://www.freerepublic.com/focus/f-news/3653607/posts
And also sure enough, there is a 45 day time window for Congress, which comes AFTER the special message is transmitted.
The timeline for approval by both the house and senate is 45 days.
You assigned May 8 as the end date for that 45 days. The house and senate have not approved the rescission request. Looks like it fails, eh? Just kidding.
If there is a deadline for the president to submit the special message, I'd like to be referred to the specific clause in the statute that imposes that deadline. As far as I know, Trump can send another special message relating to rescission, even though the appropriations are more than 45 days old.