After reciting a blessing on spices (besamim), how much time must elapse before a new blessing is required? Or is this not time-dependent, but rather something that depends on one's attention (i.e. until hesech ha-daas)?
1)One who recites a Bracha on a pleasant smelling item and puts it down with the intent of not smelling it anymore, and then wants to smell it again , must recite a new Bracha, as the intent to put it away is considered an interruption (Hesech Hada'as)
If, however, his/her intent was indeed to pick it up again soon and smell it again, no new Bracha will be required. (Shulchan Aruch Siman 217:1)
Even if he/she left the room, as long as there was intent to return shortly and smell it again, no new Bracha will be required. (Mishna Berura 217:6 and Sha'ar HaTzion 217:11 and 12)
2) If one was away for a long period of time, went to Shul to daven, went to work, or took a nap, a new Bracha will be required. (Mishna Berura ibid. see also Ketzos HaShulchan Siman 62:19)
Thus, on Yom Kippur when it is customary to smell Besamim in Shul throughout the day, only one Bracha is recited, and that Bracha will suffice for as long as one is in Shul.(See Mishna Berura Siman 46:14 and Siman 612:18 . See also Taz Siman 639:20 , Mateh Efraim Siman 622:3 and Elef L’mateh there S”K 2, Kitzur Shulchan Aruch 133:20 and Shu”t Shevet HakeHasi Vol. 2 Siman 98 for more on this topic)
Purposely taking your mind off the Besamim, in order to obligate yourself to recite a new Bracha each time (as many people do on Yom Kippur in order to use the Bracha on Besamim as a way to reach the 100 Brachos needed daily, which is difficult to accomplish on Yom Kippur) should not be done, and is considered a Bracha Sh'Aina Tzericha, an unnecessary Bracha, and is forbidden. (Mishna Berura Siman 612:18. Parenthetically, See Sha'ar HaTzion 556:1 that on Tisha B'Av Besamim should not be smelled, as it is a pleasurable activity which is forbidden on Tisha B'Av)