Substition on Kickoff

[kd0254]

Friday night Team K lines up to kickoff following a score and extra break for hydration timeout. K lines up with 12 guys, they are my sideline so I have it right away. About 3-5 seconds goes by and just before the RFP whistle I flag K for illegal substitution.

Head coach asks "don't I have 25 seconds after the whistle" and I stated "replaced player has to leave the field immediately (which is now 3 seconds)".

You guys have a flag when I threw it or do you wait longer to flag (i.e. after the RFP). I know one thing that I would not let this turn into IP by waiting.