As far as I know, it tooks 9 months to get a baby created. However, you can die at any time. What you think would happen if, when the Avatar dies, there were no "empty babies" in the next nation of the cycle for Raava to get in? For example, a Fire Avatar dies but can't reincarnate because all the air nomads are adults and there's no babies being born at the time of the Avatar's death. What you think would happen? Raava would just fly around until she finds a suitable next Avatar? Furthermore, what you think would Raava do if an entire nation became wiped out of existence? Would she skip to the next nation? Would she recreate the said nation by giving random people bending abilities, like the harmonic thing?