Entirely depends on your application. Some students need 1 year (~30 units) of improved grades; other weaker students, or if complete career changers with no prereqs, will probably require 2 years (~60 units).
From what I hear, your postbacc should honestly not be more than 2 years unless you're doing it part-time (due to diminishing returns on each additional class). If you're still a weak applicant after competing 2 years of mostly A's, you probably need to do an SMP, or reconsider the career path.