How long does it take a Blood Moon to show up and what respawns exactly after the event?

Solution 1:

1: Blood Moon spawn rate is tied to the number of enemies that have been killed, modified by a seemingly random value. It has no fixed spawn rate, but can glitch to happen very often, and not at midnight. This can be fixed (reportedly) by completely closing the game, removing the cartridge from the slot, then reinserting it.

2: As said above, it doesn't happen at a fixed rate so can't be tracked in real minutes.

3: 1 second in real life equals 1 minute in game time. It takes 24 minutes for a full in game day to take place. This means it takes 16 minutes for a day cycle (5AM to 9PM) and 8 minutes for a night cycle (9PM to 5AM).

4: No, apart from the glitch, which doesn't count as an event

5: All weapons/shields/bows/materials/ore rocks that appear in the overworld, as well as monster camp chests. Shrine enemies will respawn (including "Test of Strength" guardians), but shrine chests will not.

Solution 2:

It seems there's tons of misinformation on how blood moons actually work, which is unfortunate. The current accepted answer is totally wrong for questions 1 and 2. Blood Moons have absolutely nothing to do with enemy kills. They are simply based on a timer.

A few days ago I posted a pretty in-depth explanation of how time, blood moon and Lord of the Mountain spawns work here. Here are the relevant parts about the Blood Moon scheduling mechanism:

[...] It turns out that time, Blood Moons and the LotM are all handled by the same system (which I'll call the time manager) and based on time, so first let me clarify how time works.

How time works in BotW

Internally, time of day is stored as a float in the [0.0, 360.0] range. [...]

Every game tick

[...] if there is no active event/cutscene:

• The game timer is incremented by 0.0083333 * elapsed frames. Effectively, this means that 1 in-game minute = 1 real-life second.
• [...]
• If the timer reaches 360.0 (midnight), the game performs special checks. See the next section.
• A value called the blood moon timer is also incremented by 0.0083333 * elapsed frames.

[...]

Every time the timer reaches 360.0 (midnight)

[...]

If a Blood Moon is scheduled:

If Blood Moons are "prohibited" (see below for a full list of conditions), the timer is set to 2880.0 and nothing happens. Otherwise, the Blood Moon cutscene (Demo011_0) is triggered and the timer is reset to 0.0.

The game then determines whether to schedule a Blood Moon or not:

If the 'FirstTouchdown' flag is set (i.e. if the player has left the Great Plateau), and if the Blood Moon timer is > 2520.0 (7 in-game days):

• The WM_BloodyDay flag is set to true, which means that a Blood Moon is scheduled for the next night.
• The Blood Moon timer is reset to 0.0.

Otherwise, the "Bloody end reserve timer" is set to 150. This ensures that WM_BloodyDay is cleared 5 in-game minutes (= 5 seconds in real life) after a Blood Moon occurs.

Blood Moon inhibitors

If any of the following conditions are true, the Blood Moon cutscene will not be triggered.

• Not on MainField (main overworld)
• IsInHyruleCastleArea is set
• LastBossGanonBeastGenerateFlag is set (fighting Dark Beast Ganon)
• BloodyMoonProhibition is set
• Wind_Relic_BattleStart is set (Vah Medoh battle)
• Electric_Relic_Battle is set (Vah Naboris battle)
• Water_Relic_BattleTime is set (Vah Ruta battle)
• Something involving the Sky Manager

However, these do not prevent the Blood Moon timer from advancing, or the WM_BloodyDay flag from being set. [...]

Takeaway

Time starts at 05:15, not 11:00, and flows normally until 11:00 is reached. Activating the Great Plateau Tower sets the time to 11:00, shows the time on the UI and restores the normal flow of time.

Blood Moons occur at midnight if and only if the WM_BloodyDay flag is set. As far as I can tell, there is no way to force blood moons to be scheduled.

That flag is set the night before at midnight if you have left the Plateau and if the Blood Moon timer has reached 7 in-game days. Not 7 days and 15 minutes, but exactly 7 days (2520/360 = 7). Also, note that you do not need to pass time at a campfire after the WM_BloodyDay flag is set.

The Blood Moon timer starts ticking as soon as you've left the Shrine of Resurrection normally. Any time spent in a state where even the main game timer is paused does not count. However, you do not necessarily need to be in the open world: time spent in shrines, divine beasts, etc. does count.

Passing time at a campfire or sleeping repeatedly does not help trigger a Blood Moon at all. Quite the opposite, as the timer does not advance during cutscenes.

Let me be clear: killing enemies has absolutely no effect on blood moons. The time manager code does not even check for enemy kills... which proves that it is just misinformation.