When one removes their tzitzis at night to go shower, is there any reason to put them back on before going back to shul for maariv? Also, if one takes a haircut and bath during the day and has a hefsek between taking off their tzitzis and putting them back on for around 45 minutes should one make a bracha? How long of break is considered a hefsek for the talis katan?
There is no need (or reason) to wear tzitzis at night after having taken them off (though some wear tzitzis even to sleep, based on Arizal–see Mishnah Berurah 21:15).
A haircut and bath that takes around 45 minutes, would constitute a hefsek, and you could make a new berachah when putting the tzitzis back on. However, it is not common practice to make a berachah, unless the hefsek is unusually long, for instance a two-hour game of tennis. Note that Sephardim would make a new berachah even if the tzitzis are worn again without delay. See Shulchan Aruch (8:14); Mishnah Berurah (8:37) and Biur Halachah, who recommends that when reciting the berachah in the morning, one should have intention that the berachah only covers the tiem up to the time when one takes off one’s tzitzis for the bath, thereby avoiding any doubts.