There are approximately 120,000,000,000,000,000,000,000,000,000,000,000,000,000,000 possible addresses that start with 1bitco (that's 1.2 X 10⁴⁴).

There are approximately 80,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 possible addresses that do NOT start with 1bitco (that's 8 X 10⁵²).

So, for every address that you find that starts with 1bitco, you need to search through on average approximately 666,666,667 addresses.