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14 days. See the Tier 5 policy guidance, paragraph 85:
If your leave was curtailed to 24 December 2018 (end of internship + 14 days), leaving on 7 January 2019 would be an overstay of 14 days. This would not have any specific effect on any future immigration application for the UK, but it would need to be declared on any application.
I don't think there is anything to worry about anyway, whether your leave has been curtailed behind the scenes or not.
They make a distinction between when someone "has ceased work or study" (see page 58 of the curtailment guidance) and when "the employment, volunteering, training or job shadowing will end earlier than originally expected" (page 59). The former is curtailed to 60 days, the latter is curtailed to the new end of the work plus the 14-day wrap-up period.
Unless I misunderstood, OP is saying they quit the internship on 10th December and left the on 7th January.
My former Sponsor reported the HO in December 10th and I left on January 7th.
Thank you so much for answering my doubts CR001, I very much appreciate it. My visa expires in March 23h.