You can use it to talk about how long it will take you or someone else to do something.
¿Vas a tardar mucho?
Are you going to take a long time?
Tardé una hora en escribir la carta
I took an hour to write the letter
Voy y vengo, no tardo mucho
I'm going and coming right back, I won't be long
Don't take too long
You can use it to talk about how long it takes to get somewhere.
¿Cuánto tiempo tarda en llegar al aeropuerto?
How long does it take to get to the airport?
¿Tarda mucho en llegar?
Does it take long to get there?
Tarda dos horas en llegar
It takes two hours to get there
¿Cuánto tiempo va a tardar mi orden?
How long is my order going to take?
And you can use tardar to talk about how something takes in general.
¿Cuánto tiempo va a tardar?
How long is it going to take?
And of course you can tell someone they took too long to do something.
How long did it take you to think of that answer?
You took a long time!
There's also the expression A más tardar.
Te pago a más tardar en tres días
I'll pay you in three days at the latest
And there you have it. Everything you need to know start using the verb tardar like a native Spanish speaker.
That's it for today, ¡Hasta la próxima!