How long must a widow or divorcee wait to remarry?

A. Not pregnant and not after birth:[1]

All widows or divorced women must wait 90 days from after the divorce, or death of the husband, prior to remarrying.[2] This applies even if the woman is not capable of bearing children, or did not have relations with her husband. The 90 days are counted beginning from the day after the divorce or death of the husband. On the 91^st day she may get married. 

Shidduchim:[3] The above women may date and become non-Halachicly engaged [no Eirusin] prior to the 90 days.

B. Pregnant or after birth:[4]   

A divorcee or widow that is pregnant or within 24 months from after birth, must wait until the baby is 24 months old in order to remarry. This applies even if she is no longer nursing her child. The 24 months is counted beginning from the day after birth of the child. After the 24 months pass she may remarry.[5] If there is a double Adar within these 24 months, she is to initially be stringent and not consider it as part of the 24 months, and hence wait a total of 25 months.[6] The above law however carries the following exceptions, in which case she must only wait 90 days as is the law by any other widow or divorcee:

1. If the child passed away she may remarry right away, so long as 90 days have passed since the divorce or death of the husband.
2. If the child was weaned before the divorce or death, she may remarry right away, so long as 90 days have passed since the divorce or death.
3. If her milk dried up before the divorce or death, she may remarry right away, so long as 90 days have passed since the divorce or death.