Like Jake said, the slants wil not grow at the same rate as a smack pack. Smack packs have at least 100 billion cells to start with, and the purpose of a starter is to provide enough wort for them to grow and double that number. The slants are several log scales fewer in total cell numbers, and will take much longer to grow to the same cell density. Depending on the age, temperature and total volume, starters made from slants will take 3-5 days to show activity and a few more to reach maximum cell density.
Like I have said before, you cannot make starter from slants the same way they are made from smack packs, they are two completely different situations. When making a starter from a slant start a week in advance, and cold crash before pitching.