Yes, there’s no time limit. Time is the MOST important factor if someone is missing and the belief that you have to wait X days is very dangerous.

Theres a lot of messed up stuff with this situation (most notably the threat that it’ll continue past turning 18), but someone with parental authority over a minor who earnestly believes the minor to be missing should call the police, they should not be waiting.

(Whether the grandparents earnestly believed that in this situation is another question entirely)