I want to implement three effects which display every 5 seconds. I know that using delay() is not a good method so I want to use milis(). Duration times of the effects are different from each other. How can I assure "non-freezing" or displaying 2 or 3 effects at the same time? Should I measure the time of display of every effect and on its base adjust intervals or is there another solution? What do you think?

EDIT:

Here is my code:

void loop() {
   unsigned long currentMillis = millis();
    
   if(currentMillis - previousMillis >= interval) {
     previousMillis = currentMillis;
     i++;
    
    switch (i){
      case 1: 
        meteorRain(0xff,0xff,0xff,10, 64, true, 30); // efekt spadającej gwiazdy
        Serial.print("meteorrain: ");
        Serial.print(previousMillis);
        Serial.print("\n");
        break;
        
      case 2: 
         colorWipe(0x90,0x90,0x90, 50);
         colorWipe(0x00,0x00,0x00, 50);
         Serial.print("colorwipe: ");
        Serial.print(previousMillis);
        Serial.print("\n");
         break;

      case 3:
         MyEffect();   
         Serial.print("My effect: ");
        Serial.print(previousMillis);
        Serial.print("\n");
         break;
    }      
    if (i>3) i=0;
    }
}

I've ommited the code of effects (it doesn't matter, I think).