there is no set answer to your question as groth rate will depend on a number of things ,it will grow more if it is happy were it is, the weather plays a part ,it grows to betwen five and ten meaters to maturity, your bush or smal tree is a subpecies of siebold's magnolia, or oyama magnolia,, it is the hardest of all the magnolia that is all I can do for you I hope it helps,

Many thanks for the info