Answer: There are many varieties of camellia, with different mature sizes. The mature size is often on the nursery tag, or the nursery where you bought them should be able to tell you. Spacing should be based on the plant's mature size, so they have room to grow without being crowded (lack of air circulation can promote fungal diseases, such as powdery mildew) or needing constant pruning, which is detrimental to the long-term health of the plant. Camellia japonica, a commonly-sold variety can reach 20 feet high and wide, but more often grows to about 6-12 feet high and wide.
How fast they grow depends on the variety, the health of the plant and growing conditions, such as sun, wind, soil fertility, etc., so it's difficult to pinpoint a time. I'd check with the nursery where you bought them for details on your variety. Good luck!
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