The embattled premier's legal team was supposed to present its case by Friday but asked for a 45-day extension because of the amount of evidence it needed to collect, the country's National Anti-Corruption Commission (NACC) said in a statement.
They were granted a shorter extension of 15 days, starting from Friday, the statement said.
The NACC filed charges against Yingluck in February, saying she had ignored warnings that a rice subsidy scheme was fostering corruption and causing financial losses.
The rice scheme, which guarantees farmers above-market rates for their produce, has become a lightning rod for anger among anti-government protesters.
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The government has faced months of mass protests calling for Yingluck to step aside in favour of an unelected "people's council" to tackle what opponents see as a culture of money-driven politics.
The backdrop is a longstanding struggle between a royalist establishment, backed by the judiciary and the military, and Yingluck's billionaire family which has strong support in the northern half of Thailand.
Her administration has limited caretaker powers because opposition demonstrators disrupted a general election last month.
The authorities have also suffered a series of legal defeats by the courts, which have been accused by government supporters of colluding with the opposition to try to oust Yingluck.