Am I required to repeat after the muezzin every time I hear him?

The commands to repeat the Adhan are based on the Prophet's (peace be upon him) saying: "When you hear the Mu'adhdhin, say as he says." This command is for recommendation (istihbab), not obligation (wujub).

Several matters indicate its recommendatory nature: - The Prophet's (peace be upon him) saying when he heard a Mu'adhdhin calling the Adhan: "He is upon the natural disposition (fitra)" and "He has exited the Fire." This is understood to mean that the command is not obligatory. - What is reported in Al-Muwatta' that the Companions used to speak when hearing the Adhan, and Ibn Sa'd reported from Musa bin Talha that Uthman used to speak when hearing the Adhan. It is also reported from Tha'laba bin Abi Malik that they used to speak on Friday while Umar was sitting on the minbar. Had following the Adhan been obligatory, they would not have done so, and there would have been a report from the Prophet (peace be upon him) indicating its obligation.

The virtue of following the Adhan is immense: - Whoever says what the Mu'adhdhin says and then says: "(There is) no god but Allah from his heart, will enter Paradise." Narrated by Muslim (385). - Whoever says when he hears the Mu'adhdhin: "I bear witness that there is no god but Allah, alone without partner, and that Muhammad is His servant and His messenger. I am pleased with Allah as my Lord, with Muhammad as my messenger, and with Islam as my religion, his sins will be forgiven." Narrated by Muslim (386).