How do blood replacements work, and how long can the player be off?

Law 3.25:

• A player with a blood injury leaves the field and may be temporarily replaced.
• The player returns as soon as the bleeding is controlled and/or covered.
• If the player cannot return within 15 minutes (actual time) of leaving the playing area, the replacement becomes permanent.

Other points:

• A temporary replacement can itself be temporarily replaced, even if all replacements have been used (Law 3.28).
• If the 15 minutes run out during half-time, the replacement becomes permanent unless the player returns immediately at the start of the second half (Law 3.32).
• If the temporary replacement is yellow-carded, the original player may not return until the suspension ends (Law 3.31). If the temporary replacement is sent off, the original player, if medically cleared, is treated as tactically replaced (Law 3.30).
• A tactically replaced player may come back on to replace a player with a blood injury (Law 3.33).