How do I signal an individual recall, and how long must flag X stay up?

Under rule 29.1, when at a boat's starting signal any part of her hull is on the course side of the starting line (or she must comply with rule 30.1, the I flag rule), the race committee must promptly display flag X with one sound.

The flag must be displayed until the hull of each such boat has been completely on the pre-start side of the starting line or one of its extensions, and until all such boats have complied with rule 30.1 if it applies. It must come down no later than four minutes after the starting signal or one minute before any later starting signal, whichever is earlier.

Rule 29.1 does not apply if rule 29.2 (general recall), 30.3 (U flag) or 30.4 (black flag) applies.

Source: Racing Rules of Sailing 2025-2028, rule 29.1, https://cdn.revolutionise.com.au/site/ofeyzh6cn3tumfga.pdf