The deadline is five business days after crossing the threshold, with amendments required promptly on material changes such as a 1% move in the position. The purpose section is the valuable part: it is where an activist-investor states what it intends to push for.
A 13D is distinct from a passive schedule-13g. Switching from 13G to 13D on an existing stake is a declaration of intent and often the real news.
Example: a fund crosses 5% on 3 March and files on 10 March disclosing 6.8% and a demand to explore a sale. The stock rises 9% on the filing, which is the market pricing the possibility of a deal-premium.
Related: schedule-13g, activist-investor, proxy-fight, form-13f, tender-offer