A Hong Kong employment visa is issued for 12 months. Later, the immigrations department will renew the visa on 2-year basis until you reach the 7 years required for application of permanent residence. On the seventh year, you will be eligible to apply for permanent residence that does not require sponsorship or fear of deportation. Your dependants will also get the same status to live in Hong Kong.
At this point, you will be free to seek employment in other companies without having to notify the immigrations department. Note that you are required to have an identity card immediately after securing an employment visa.