If you want to get a fiancé visa then you need to prove that you are a US citizen and you also need to get married within 90 days of your partner entering the United States. You should also note that any previous marriages must have been terminated before the fiancé visa marriage can go through and you must have seen one another within 2 years of filing your marriage petition. There are however two ways that this can be waivered, the first would be if the requirements of this would affect the long standing customs of your partner's social or foreign culture. The second would be if the requirements would be of extreme hardship to you or your partner.
Once the K-1 has been issued, your fiancé would be allowed to enter the United States within the next 90 days. Once the two of you are married, you may then apply for a permanent residency for your partner. They would be allowed to stay in the States until the application has been processed by USCIS.
If your partner has children under the age of 21 then this should be mentioned in the I-129F form. After admission, the partner may also apply for work; however the form I-765 would need to be filed as well. If, for some reason, you do not get married in 90 days then your duration can't be extended. Instead, your fiancé would need to leave the United States at the end of the 90 days. If the partner does not leave after this time then they would be in direct violation of the immigrant law and this may result in removal from the States. If the marriage ceremony has been completed within 90 days however, this would no longer be an issue as you would be legally married.