How long should i engage the starter to start a car ? After how much time should i try 2nd time to start a car?

Crank for 5-10 seconds and then give the starter a couple of minutes to cool off. With a good battery you could probably try 20 times but if it doesn't start after 3-4 attempts you're probably only wasting your time and could be doing a lot of damage. Find out what the problem is.

All depends on your battery as far as how long you should try, and what your mechanical problem is as to why its not starting. Just remember, your starter engages the flywheel. The flywheel has many teeth around it, so allow it to come to a stop before trying to engage it again. That loud "kkkkkkrrrrrrrrickkaaadfhfhhsd" when tapping the ignition while a car is on or too soon when trying to start is your starter trying to engage the teeth that are already in motion, not good. If its a fuel pump prob, just turn key to on and let pump prime well before start.

it should start after half to 1 1/2 turns. some conditions may require a few revolutions. do not burn out the starter by over heating it. hot parts wear out quicker than normal temperatures.

Short bursts of attempts works better. Find out for you model whether you should leave the gas pedal up or down when doing this to keep from flooding it. Listen to the turnover rate when it starts to slow down give it a rest. Along rest.