How to convert Litecoin difficulty of 6 to an expected time per block solved? The current difficulty for mining Litecoins is about 6. If I mine them at 25kh/s, how often should I expect to solve a block with a difficulty of 6?

I'd also appreciate an explanation (or formula) of how the calculation is done.
I don't know much about Litecoin, but if its difficulty works the same as in Bitcoin, then on average it takes approximately difficulty * 2^32 hashes to solve each block.

Since you're calculating 25000 hashes per second, it should take you around 6 * 2^32 / 25000 seconds to solve a block.  Or about 286 hours.

Does that sound about right?