Auto Submit form after entering 20 digits

[xLuca94]

i want to auto submit the form, after entering 20 digits in input filed.CODE (the form i'm using)\[code\] <form id="inter_result_form" action="<?php echo $_SERVER['PHP_SELF']; ?>" method="post" onsubmit="return vali()" > <input type="text" name="rollno" id="rollno" maxlength="20" autofocus="autofocus" value="http://stackoverflow.com/questions/15716536/<?php echo $_REQUEST["phoneno"]; ?>" /> <input type="submit" name="submit" value="http://stackoverflow.com/questions/15716536/Submit" id="submit" /> </form>\[/code\]