When should fund managers file as Exempt Reporting Advisors (in the US)?

Generally speaking, ERAs must file the Form ADV within 60 calendar days of becoming an ERA - thus they generally must file within 60 calendar days of a fund's first closing, in most cases.

Please see the following (Item 13 in particular) for more information:

https://www.sec.gov/about/forms/formadv-instructions.pdf