The gestation period of Russian hamsters is 18-21 days and if mating was observed then the time of the expected arrival of the litter can be roughly calculated. If mating was not observed then it is often not possible to know tell when the litter is expected. Many females do not appear pregnant until a couple of days before the birth but an increased aggressiveness of the female towards her mate, colony hamsters or humans is often an indication of pregnancy. The female will often banish the male or other hamsters from the nest a couple of days before pregnancy and/or after the birth. As soon as it is noticed the female is pregnant it is best to clean the cage.
It is not necessary to remove the pregnant female from the pair or group as the male and other hamsters in a group will help her to rear the young. Also keeping the female within the pair or group will ensure the baby hamsters learn how to interact with other hamsters as they grow up.
However, the female often comes into season soon after giving birth so the male will often mate with the female again so if it is not intended to breed further litters the male and female should be separated prior to the birth.
Dwarf Russian hamsters have an amazing ability to delay implantation, meaning that mating can take place but implantation of all or some of the fertilised eggs can be withheld by the female until a later date and so even if mating takes place the female may not give birth until some time much later. In this way the female can regulate her breeding even if kept in a mixed sex pair or group.