If one makes borei basamim on shabbos or fast days: How long should one wait between smelling it numerous times for a new bracha to be made?
A new blessing can be made after a ‘hesech ha’daas,’ meaning after a person turns to other matters and forgets about smelling scents.
If, for instance, a person sits at a dest with fragrant leaves, a single blessing can last him the whole day, because there is no hesech ha’daas from smelling the fragrance. If, however, a person goes to daven, or does something else that causes him to forget about the scent, a new berachah is made upon smelling again.
Sources: See Mishnah Berurah 217:6; Shaar Ha’tzion 10.