I haven’t gotten my xbox live gold membership extended. I bought the Master Chief Collection day one and have logged over 90 hours into it. I contacted xbox support and they told me to post this here and someone from halo support would contact me and extend my membership.
It hasn’t rolled out to everyone yet, but should over the next few days. The drop dates were mid to late January, so if you don’t have it by the end of January I would contact support again. If you have a good subscription, check http://commerce.microsoft.com and you can see if it has been extended. Some folk have noted they got an xbox live message with details, I had my account adjusted a day or so ago but have not had any messages