Disclaimer: I used l0phtcrack 15 years ago, but I'm not familiar with the current version.
Do you use a weak/short Administrator password? If not, then don't expect it to get a result in half an hour. As you know your password, you can calculate how long it would take to break it. Also, just define the correct character set that you are using for the Administrator account and then let l0phtcrack run through (finish).
Another important point to be aware of: In Windows 8 there is no Administrator. When you setup Windows 8, it asks you to create a user (which gets Admin rights), but it is not "the" Administrator. When you go to the User Accounts dialog, you only see users, the Administrator itself is no longer listed. Actually the real Administrator still exists, but it is not listed there. You can still see the real user list when you go to Computer Management, Local Users and Groups. I assume the real Administrator will get created with a very strong random password (or empty?) and you just don't see it. This very strong password (still my assumption) will not be possible to crack by using l0phtcrack or any similar tool, even if you let it run forever. If I look at the users (in Computer Management), I can see that the users Guest and Administrator are disabled, which kind of proves that there's nothing to calculate for them.