How to delay or slowly loop commands?

For functions there is a built-in way to do this:

/schedule function <function_name> <time>

So if you for example want to execute the function named "test" in the namespace "abc" in 1 hour, which equals 3 in-game days, you can use any of these commands:

/schedule function abc:test 3d
/schedule function abc:test 3600s
/schedule function abc:test 72000t
/schedule function abc:test 72000
/schedule function abc:test 3d replace
…

Unit postfixes for minutes, hours etc. do not exist. The in-game days unit is not affected by sleeping or /time.

If you have changed your mind, you can abort a scheduled function execution like this:

/schedule clear abc:test

If you want to schedule multiple executions, you can use append instead of replace. If you want to replace all existing schedules of this function with the current one, you can use replace or just provide no mode at all, replace is the default.

To loop execution, you can simply use the /schedule command at the end of your function. Usually you'll want to do this under some condition, so that it doesn't keep looping forever.

If you cannot or do not want to use functions or /schedule, you can still use the old scoreboard timer method. This needs a little bit more resources, because it actively does something every tick, but it shouldn't cause noticeable lag.

As a preparation, you need a scoreboard:

/scoreboard objectives add timer dummy

The "dummy" type is one that is not affected by anything except commands.

In a repeating command block or ticked function, execute this command:

scoreboard players add $timer timer 1

The name "$timer" cannot possibly be a real player name, so it is a good choice for a dummy player name. If you do not want it to show up in a scoreboard sidebar display, start the name with a # character.

Now you can do something once the timer reaches your desired number of ticks like this:

execute if score $timer timer matches 100 run say 5 seconds are over!

If you want to do something repeatedly after 5 seconds, simply replace 100 with 100... If you want a variable starting time, compared to someone else's score, you can for example use >= @p points instead of matches 100.

If you want to repeat something every x seconds/minutes/…, you can just reset the timer after you have done whatever you want to do with it. So at the end of your command block chain or ticked function, you put this:

execute if score $timer timer matches 100 run scoreboard players set $timer timer 0

This concept can also be used to do something every x times something happens. To do this, simply increase the timer only conditionally instead of every tick, for example like this:

execute if block ~ ~1 ~ stone run scoreboard players add $timer timer 1

But make sure to change whatever the command is checking for directly afterwards, otherwise it keeps ticking up by 1 every tick as long as the condition is matched.

Area Effect Cloud method

If you are unwilling to set up a scoreboard because you think excess scoreboards are a waste of space, a better method would be the Area Effect Cloud method.

Note that this does not work for functions. Functions should use /schedule to schedule another function.

Area effect clouds (AECs) have a Duration tag and an Age tag. Their Age will increase at a rate of 1 per tick. When its Age gets to its Duration, the AEC will disappear.

The following command will summon an AEC that disappears in 100 ticks (5 seconds), because an unspecified Duration defaults to 0:

summon minecraft:area_effect_cloud ~ ~ ~ {Age:-100,Particle:"take",Tags:["delay"]} 

So now that we can make an AEC that hangs around, how can we make it execute something after the delay is over?

The best way to do this is to detect when the AEC is about to expire, and make it activate a command block. We can choose to summon the AEC inside the command block we want it to activate, and then when it is about to expire, flick its auto tag to 1b, setting it to Always Active, and then flick it back to 0b. The following commands will do just that:

tag @e[type=area_effect_cloud,tag=delay,nbt={Age:0}] add delayActivating
execute at @e[type=area_effect_cloud,tag=delayActivating] run data merge block ~ ~ ~ {auto:1b}
execute at @e[type=area_effect_cloud,tag=delayActivating] run data merge block ~ ~ ~ {auto:0b}

That's the implementation of the system that makes it work. Then, to initiate a delay, use this command:

summon area_effect_cloud _________ {Age:-_____,Particle:"take",Tags:["delay"]}

Along with the other commands, this command will work to start a delay. This basically will activate the command block at the position you set, after a delay of however many ticks you set.