Addition to Alex' answer:
$(function() {
$('input[type="submit"]').prop('disabled', true);
$('#check').on('input', function(e) {
if(this.value.length === 6) {
$('input[type="submit"]').prop('disabled', false);
} else {
$('input[type="submit"]').prop('disabled', true);
}
});
});
<input type="text" maxlength="6" id="check" data-minlength="6" /><br />
<input type="submit" value="send" />
But: You should always remember to validate the user input on the server side again. The user could modify the local HTML or disable JavaScript.