While most Russian dating websites are genuine, a number of them are fake and their so-called members are not real either.
They will ask you for large amounts of money, promising to set you up with a beautiful Russian women but your money is as good as gone because the agency is a scam.
The period from first contact to first cash request could be a few days or a few months but regardless how long you've "known" the person, a request for money is a scam.
Scammers operate from countries with low per capita income and even if a scam takes months to pay out it is worthwhile for them, even for just a few hundred dollars.
Choose a reputable dating service or agency in order to avoid being scammed this way.