Martha Stewart recommends soaking the bread for about 10 minutes on one side, flipping it over, and soaking it for an additional 10 for even absorption throughout the entire slice.
Taking your bread out of the liquid mix before that time has elapsed may not allow the bread to soak up enough and get the desired interior texture, especially with thick slices.
After soaking, cook and caramelize the sweet, eggy exterior. A quick dip in the mix could leave your French toast with a slightly eggy outside but a drier, less flavorful interior.