I've seen people "thrown in at day one", and people who do kihon for a long time before kumite. The places where I've trained, students can opt to do kumite or not, their choice. So far, my observation (from a limited sample) is that the early starters seem to do better in kumite. However, I don't know if this is because the timing makes a difference, or because different types of people opt to start early.

One frustrating observation is that some students do beautifully in kihon but it doesn't translate at all to their movements in kumite, even when minutes apart.
God grant me a good sword and no use for it. -- Polish proverb