Staffan de Mistura said participants would begin arriving in the coming days and that he would be having some informal talks over the weekend.
"But the substantive deeper part of it... will be on Monday," he said, saying the negotiations would "last not beyond March 24th", when there would be a break.
"There will [be] a recess of a few days, a week perhaps, ten days" before the talks resume, he said.
"Having a timetable and a time limit is healthy for everyone."
The UN is hoping to restart peace talks that collapsed last month, building on a ceasefire that has led to the first significant decline in violence in Syria's nearly five-year civil war.
The truce between President Bashar al-Assad's regime and non-jihadist rebels is part of the biggest diplomatic effort yet to resolve Syria's conflict, which has killed more than 270,000 people and displaced millions.
The partial truce, which was negotiated by Washington and Moscow and which does not apply to the Islamic State group or the Al-Qaeda-affiliated Al-Nusra Front, has largely held since it began on February 27th.
"The cessation of hostilities... is still holding, and it is making a direct impact on the lives of millions of Syrians inside the country," UN humanitarian coordinator for Syria Yacoub El Hillo told reporters.
As in the previous round, the negotiations will take the form of "proximity talks" with de Mistura shuttling between the different sides.
The Syrian regime on Monday confirmed it would attend the Geneva talks, while the opposition has said it was still considering the matter despite a major lull in fighting.
A small delegation from the Riyadh-based High Negotiations Committee was meanwhile expected to attend a meeting in Geneva Wednesday afternoon of the international task force overseeing the ceasefire.
Six key areas unreached
Wednesday morning, there was a meeting of another task force monitoring efforts to increase humanitarian aid deliveries to nearly half million people in besieged areas and another four million in hard-to-reach areas.
In the past four weeks, 536 trucks had reached 238,845 people, 150,000 of them in besieged areas, de Mistura said.
His special adviser Jan Egeland meanwhile hailed the fact that ten out of 18 besieged areas had been reached, some with multiple convoys.
"The bad news is that we still have not reached six important besieged areas," Egeland said, referring to areas such as Daraya, Douma, besieged by government forces, and Deir Ezzor where some 200,000 people are under siege by IS jihadists.
The aim is to reach a total of 870,000 people in hard-to-reach areas by the end of April, el Hillo said.
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