when do i change the spark plug

[prayank]

how important is it to change the spark plugs

how will i know is it about time.

[vivek]

Spark plug changes is one of the most important part of your car and it should be change every 30k - 40k km. If its too old or getting bad you may have many issues;

Misfiring

Pining engine

Bad gas mileage

difficult start

excessive smoke etc.