I want to make my creature disappear after 1 minute. but how???

The easiest method would be to just give it an invisible potion effect, (say, Luck, with particles disabled), and then on the entity's update tick, despawn it if it no longer has that potion effect. (You could do something similar with an NBT tag, but it's the same basic idea.)

...I mean, sure, but it is literally just those two steps. Here's a more detailed explanation:

• Using the 'on initial entity spawned' trigger for your entity, make a new procedure.
• In this procedure, use the 'give entity potion effect' to give your entity 1200 ticks of luck, with particles disabled. 
• Then, using the 'on entity update tick' trigger for your entity, make another new procedure.
• In this procedure, add an if bracket, and attach a 'NOT' function to it. Have the if bracket check if your entity has luck, (using the 'does entity have potion effect' function.) Because of the Not function, the if bracket will now trigger if your entity does not have luck. You could also check if the remaining ticks of luck = 0.
• Then, inside the if bracket, add the 'despawn entity' function. So now, evey update tick, if your entity does not have luck, it will despawn.

Nice explanation Mindthemoods! :)

You can use this as an alternative, i might be wrong tho.

Note: Oulined block converts seconds (60) to ticks (1 second = 20 ticks)
You should put this in "On initial spawn" trigger in your living entity so countdown starts when it spawns.

I haven't noticed the topic was made month ago, im sorry for necroing this topic