durar vs tardar
Both measure time. Durar is how long something lasts. Tardar is how long it takes to get done or arrive.
durar
To last — how long something continues.
La película dura dos horas.
The film lasts two hours.
The event has a length.
Esta batería dura todo el día.
This battery lasts all day.
Lifespan of an object.
Nada dura para siempre.
Nothing lasts forever.
General truth.
tardar
To take time, to be late — the time needed for a person or process to complete something.
Tardo veinte minutos en llegar.
It takes me twenty minutes to get there.
Tardar + time + en + infinitive.
¿Cuánto tardas?
How long will you be?
Asked while someone gets ready or is on the way.
No tardes.
Don't be long.
A common send-off. Also 'tarde' = late.
Tricky bit
'El viaje dura tres horas' (the journey lasts three hours) vs 'tardo tres horas en llegar' (it takes me three hours to arrive). The first describes the thing, the second the person.