At times one may recite Shacharis early and spend the rest of the morning learning Torah in the shul. If he is sitting in a shul that hosts one minyan after another, he may hear Kedusha every fifteen minutes! Must one interrupt one’s learning to answer the Kedusha each time?
Every time one says Kedusha with the congregation he fulfills a mitzva of kiddush Hashem. While a person is not obligated to actively seek out this mitzva, if he encounters an opportunity he cannot forfeit it, even a hundred times a day. As long as a person is sitting in the shul and hears Kedusha, he must join in the response (Igros Moshe 3,89).
Sometimes a person walks by a shul where the congregation is saying Kedusha. Since he is not sitting together with that congregation, he is not obligated to stop and say Kedusha with them. However, it is still praiseworthy to answer (Rema 125,2).
Text Copyright © 2011 by Rabbi Daniel Travis and Torah.org