Renewing a Family Visit Visa

I have applied for a family visit visa for 1 year and was told that this visa is only available for 6 months and I was given a visa for 6 months. Is this visa available for only 6 months or a year?

Having received a 6 month visa, I will wish to renew it before it expires. Do I need to supply all the supporting documentation (notarised translation copy of marriage certificate,etc) produced for the intial visa when I apply for a renewal, or do I just need to complete the form? What are the requirements? This information is not readily available and it is difficult to find someone at the Ministry Office to understand and answer this question. Thank you.