When filing your I-864, the NVC or USCIS will want to see proof of the cash value of your accounts. There is no telling how long you have to keep the money in your account because they very well may ask to see an updated account information sheet at the interview. You should consult with an immigration attorney to get more details on this case. You don't want to put the money in and take it out because that could be considered fraud. Consult an attorney and she'll be able to answer your question much better.
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You will need to keep the money in your account until they get their green cards.
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Talk to a lawyer. You seem not to understand the concept.
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I agree with my colleagues. There is really no set time that you need to have it there before filing the I-864. You just need to prove that it's there. To avoid fraud issues, you'll need to leave it there throughout the process.