Raising a Myrtle Nana Bonsai Tree

I have just been given a miniature Myrtle Nana tree in a pot. I presume it is a bonsai? It is the cutest little thing and stands only 12 inches high. The instructions say simply to water three times a week. I plan to keep it on my balcony, but presumably have to bring it indoors if it is very cold in winter. I have no other information on its care.

Does it flower? When should I start trimming? Can I allow it to grow into a medium sized bush?

By Ruth from Jerusalem, Israel