Last day my friend and me ware discussing one point, i.e when crawler hit the robot.txt. Lot of people say that crawler hit first .htaccesse file and then hit the robot.txt. So my question is when will crawler hit the robot.txt file.

there is no fixed time that when the Crawler will hit the Robot.txt file

its not fix that when crawler hit the robots.txt file.... its hits when crawler crawle our site.

.htaccess is NEVER requested by useragents, robots or real visitors

On Apache servers .htaccess is read by the server on each HTTP request, so if a request is made for a file called robots.txt the server will parse the directives in the .htaccess to see how the files is to be delivered in the server response.