Difficulty * 2^48 / (2^16 - 1) / Your_Hashes/s = Expected_time_to_solve_a_block_in_seconds
simplified version (not loosing much accuracy) is simpler: Difficulty * 2^32 / Your_Hashes/s = Expected_time_to_solve_a_block_in_seconds
for example:
current diff = 3815, you manage 123 MH/s rigs, then:
Expected_time_to_solve_a_block_in_seconds = 3815 * 2^32 / 123000000 = 133214s = 1.54 days
of course, if you'd manage 123 MH/s rigs you'd probably already know that :)