You have to keep them in partial sun and prune them into a tree shape and they will grow pretty fast. Depending on price i'd buy some 2fts and let em grow for a cpl of months then put em in my cages.
First, there are lots of species of _Schefflera_; let's be sure you are talking about the dwarf _S. arboricola_.
Other Sheffs may not be as safe, and that is the one you should be growing.
The biggest species grow very quickly under the right conditions; I think _S. actinophylla_ can put on over six feet ( a meter ) in a year. It would not surprise me if some other tropical species were faster.
With the dwarf Scheff, there are many varieties: some are super dwarf, some are bigger than the parent species, some are heavily variegated and have less chlorophyll. Obviously, these variety choices affect the speed of growth.
If you put up a pic of your plant ( or the one your considering ), I could give you my opinion of how it might perform.
I personally like the super dwarf forms such as 'Lussane' - they need less pruning. YMMV.