When does the worshipper raise his head from bowing if the Imam says: "Allah hears those who praise Him" after straightening up from bowing?

The place for reciting "Sami'a Allahu liman hamidah" (Allah hears him who praises Him) is when rising from bowing and standing upright, as the Prophet, peace and blessings be upon him, did. It may be difficult to guard against error in this due to frequent forgetfulness, so it might be excused.

As for the follower (ma'mum), what matters is following the Imam in actions, not words. The follower rises from his bowing after the Imam has risen, for the Imam was appointed to be followed.