find 2000 in the power of 1/3

the formula of newton referson is

$\displaystyle x_{n+1}=\frac{2}{3}(x_n+\frac{1000}{x_n^2})$

how to know what x_0 to choose for this formula

??

because when i put x_0=1 i have many steps

but when x_0=10 i have only 4

??