From the synoptic gospels, it is clear that at least the initial communities to which the gospels of both Mark and Luke originated/targeted already believed that the kosher law was no longer valid (cf Mark 7:15 and Acts 11:8-10). Since the date of composition of both have been estimated to be ca. 1st century AD, we can be confident that at least by the end of the second century these communities already did not follow the kosher laws. Unfortunately, no such passage is found in the gospel of Matthew, which is more indicative of Jewish Christians' practice (due to Matthew's target audience).

The early Church (and it was a single Church, despite not having developed specific creeds, or set a scriptural canon) decided at the first ecumenical council that all foods were clean (Acts 11), but if we look at Paul's letters we see that many early Christians, especially Jewish ones, kept the kosher laws anyway, and the early Church was alright with it. So the answer is basically "some did, some didn't."

Well that's a difficult question since communities of Christians did not have unified creeds or even a unified canon of Biblical books. The book of Acts 10:15 is usually the passage referred to by modern Christians when justifying/explaining the giving up of kosher eating rules. Luke-Acts is typically dated to 80-90 CE.

The canon, however, was not closed for several centuries, and undergoes debate even today, so take that for what it's worth.