Space in Nature is extremely limited, and so format requirements must be strictly observed, as advised by the editor handling the submission, and detailed in the manuscript formatting guide.
ensure that the paper is at the length specified by the manuscript editor (including number of figures).
Revised versions should be sent online, in accordance with the online submission requirements outlined above. They must include the manuscript reference number, a copy of the authors' response to the referees' and editors' comments, and a length estimate of revised manuscripts and figures.
It really depends. It is different for each journal. Some journals receive high levels of submissions throughout the year. Delays will occur when reviewers are busy and may have a number of journals sending them papers to be reviewed.
Some journals publish submission and acceptance dates in the first page that you may refer to.