Usually, the K-1 fiancé visa is valid for six months. However, after you enter the U.S., you can stay in valid K-1 status for a maximum of 90 days. You cannot extend your status nor switch to another immigration category. You have 90-days to get married, under U.S. immigration law, . If within the 90-day period, you decide not to go through with the marriage, you can leave the U.S. There is no penalty for doing so. You will begin to accrue unlawful presence if you remain in the United States beyond 90 days. Consequently, if you spend more than 180 days, but less than one year, after your I-94 expires, you are barred from re-entering the United States, for three years. Furthermore, you will be barred for ten years, if you stay for more than one year.
Once you overstay, you can be deported. If you encounter Immigration and Customs Enforcement (ICE) agents, you may be detained and issued a Notice to Appear in immigration court. You should not be looking to marry someone else to save you from immigration consequences. However, if you genuinely fall in love with someone and the relationship is heading towards marriage, you can win your immigration case.
However, you will not be eligible to adjust status in the United States, if you marry someone other than your K-1 sponsor. Instead, you can pursue a green card through consular processing. This means that you will have to attend the immigration interview outside the U.S.