Yevamot 41 - Three months
To avoid the problems with the uncertain parenthood that we have mentioned before , the Sages decreed that a woman should wait for three months after her husband's death, before marrying again. Since the Torah said, "... to be a God for you and to your offspring after you ," we see that one of the requirements of the covenant is to clearly distinguish between the offspring of the first husband and of the second one. The decree was extended to all cases of marriages and even betrothals, where childbearing should not be an issue - to prevent confusion.
Rabbi Yehudah disagrees: people won't be confused. So he allows, for example, a betrothed girl, whose groom died, to get married to another right away. Similarly, a woman who was married can get engaged, and then wait three months for the chuppah. The exception is a betrothed girl in the province of Yehudah, where it was known that the grooms may have been too familiar with their brides.
Rabbi Yose goes even further: there is not need to distinguish between the offspring of the first husband and the second one - something that was required by the first teacher - and everybody can get married right away; with the exception of a widow, who needs to wait for thirty days because of mourning obligation.
Art: Portrait of a Bridegroom by Antonius Heusler