Regulation 33.4.2
In addition to the three substitutions permitted by Regulation 33.4.1, each Club shall also be permitted to utilise up to two ‘concussion substitutes’ and/or two ‘additional substitutes’ (as appropriate) from those substitutes listed on the Team Sheet, strictly in accordance with the provisions of the International Football Association Board’s ‘Additional permanent concussion substitutes – Protocol B’ (‘IFAB Protocol’)
IFAB Protocol B
When a “concussion substitute” is used, the opposing team then has the option to use an “additional” substitute for any reason.
This option may be used concurrently with the “concussion substitution” made by the opposing team or at any time thereafter (except as outlined otherwise by the Laws of the Game).
So my reading is that a concussion substitute can be used at any time when concussion is suspected, and the additional substitute can be used at any point after that.