Now, I know that for solo mining bitcoins it basically isn't worth it without ASIC, but what about with litecoins? How many KHash/s should I have to effectively solo mine?
A simple calculation should give you your answer.
Current total hash rate: 11,000,000 kH/s
Your hash rate: 500 kH/s (maybe more, probably less)
Ratio of blocks found by you: 500/11,000,000 = 1/22,000. You will find 1 out of every 22,000 blocks.
A block is found every 2.5 minutes. 22,000 blocks are found in 55,000 minutes, or 38.2 days.
You will find a block and receive 50 LTC every 38.2 days on average.
It is hard to judge what is considered effective, and it is difficult to determine mathematically. With solo mining there is no pool fee; however, if you assume exponentially increasing difficulty, then constant small payments will be more than infrequent large payments.