The chronology of Acts only provides occasional points of reference for determining his timeline, and it's probable some of his narration is out of chronological order. Your question is specifically asking about the amount of time that has passed between Acts 2 and Acts 12.
The events of Acts 1-2 are presented as taking place in the few months following Jesus' crucifixion. Details for when Jesus was crucified are scant, and the issue is still being debated. What levels of certainty we have are based around historical figures mentioned in the Luke-Acts narrative. A significant starting point is Luke 3.1-2:
In the fifteenth year of the reign of Emperor Tiberius, when Pontius Pilate was governor of Judea, and Herod was tetrarch of Galilee, and his brother Philip ruler of the region of Ituraea and Trachonitis, and Lysanias ruler of Abilene, during the high-priesthood of Annas and Caiaphas ...
The easiest to define of these are:
- Tiberius Caesar, emperor of Rome 14-37 AD.
- Pontius Pilate, prefect of Rome's Judaea province 26-36 AD.
- Caiaphas, high priest in Jerusalem 18-36 AD.
Luke 3.1-2 defines the beginning of John the baptizer's mission as taking place in the 'fifteenth year' of Tiberius' rule, about 28-29 AD. Jesus' mission could potentially have begun within a year or two of John's; we do know there was some overlap between the two.
However, Luke's narration does not provide a specific duration of Jesus' mission, only that he was 'about thirty years old' at the time of his baptism (Luke 3.23), and that Pilate and Caiaphas were still alive at Jesus' crucifixion. This could potentially put Jesus' death all the way near the end of Pilate's and Caiaphas' respective rules.
The ending of Acts 12 mentions the death of Herod Agrippa. The account in Acts 12 is more or less corroborated by Josephus, in his Jewish Antiquities 19.8.2, so following Josephus' chronology, Herod Agrippa's death occurred in 44 AD.
If Jesus' crucifixion took place at the earliest possible date (c. 30 AD), Acts 2-12 covers c. 30-44 AD, a period of about fourteen years.
If Jesus' crucifixion took place at the latest possible date (c. 36 AD), Acts 2-12 covers c. 36-44 AD, a period of about eight years.