However, District Judge Wayne Salvant said he'd give the defense two weeks to make an argument against the order.
"Nothing is set in stone, so I might reconsider," he said at a court hearing in the case in Tarrant County, Texas.
The jail time is the latest twist in the deceptively complex and well-publicized sentence Couch received in 2013: 10 years of probation for intoxication manslaughter, instead of prison, after his lawyers cited a now notorious "affluenza" defense
, suggesting he was too rich and spoiled to understand the consequences of his actions.
As part of that sentence, he was always subject to possible jail time when his probation case was moved to adult court, which it did Monday when he turned 19. The situation is further convoluted by the fact that he's already in jail, accused of violating probation in part by traveling to Mexico last year.
Here's a closer look at the case:
Who is Ethan Couch?
Authorities said Couch, then 16, was drunk when the pickup he was driving plowed into four pedestrians on a road in Burleson, Texas, in June 2013. Couch's vehicle also struck a parked car, which then slid into another vehicle headed in the opposite direction.
Two people riding in the bed of the teen's pickup were tossed in the crash and severely injured. At the time, police said one was no longer able to move or talk because of a brain injury.
The case made national news after a psychologist testified that Couch was a victim of "affluenza," a product of wealthy, privileged parents who never set limits for him. A juvenile court sentenced him to 10 years of probation, despite prosecutors asking for 20 years behind bars.
Relatives of the crash victims blasted the sentence, saying they felt Couch got off too easy.
His probation was to be supervised by the juvenile court system until he turned 19, when it would be transitioned to adult court.
So why is he facing jail time now?
First, as mentioned, he's already in a Tarrant County jail after a December arrest in Mexico
. But we'll get back to that.
Wednesday's decision is technically separate from the Mexico arrest, and some sort of jail time was always possible when he turned 19 as a condition for continued probation.
State law mandates that certain young probationers serve a minimum 120 days in jail upon moving from the juvenile system to the adult system, according to prosecutors.
The maximum, Couch's attorney Scott Brown has argued, is 180 days for a transitioning probationer convicted of a second-degree felony like Couch.
So on Wednesday, two days after Couch's 19th birthday, Salvant ordered that Couch serve 720 days, or 180 days for each of the four manslaughter counts.
Couch's attorneys argued Wednesday that the four deaths should be counted as one case, not four. They'll have two weeks to try to convince Salvant.
How does Mexico come into play?
Again, Couch is already in jail. Here's how that happened:
As part of his probation, he was ordered to stay away from drugs and alcohol. But in December 2015, a video emerged that allegedly showed him at a party where alcohol was consumed.
His probation officer tried to reach him, but couldn't. A Texas warrant for Couch's arrest was issued in mid-December.
As it turned out, Couch and his mother, Tonya Couch, fled to Mexico
to avoid a probation hearing that might have led to jail time for him, authorities say. Mexican authorities found the pair later that month in a Pacific resort town and detained them.
In late January, Couch was returned to Tarrant County, where he's been in jail since.
Does the post-Mexico jailing have anything to do with Wednesday's order?
Technically, no. Tarrant County Prosecutor Riley Shaw has said the time to punish Couch for probation violations as a juvenile effectively expired when he turned 19.
However, a judge can take Couch's juvenile probation record into account when setting conditions for his adult probation, Shaw said.
After Couch serves whatever jail term Salvant decides as a probation condition, Couch will be released -- again, on probation. He wouldn't face additional jail time unless he violated the probation terms from that point forward.
Speaking of probation terms, Salvant also set those Wednesday, and they're similar to what he faced as a juvenile. Among them: He can't consume alcohol or a controlled substance, or travel beyond Tarrant and adjacent counties without permission.
What about Couch's mother?
Tonya Couch, who, like her son, was returned to the United States, is charged with hindering her son's apprehension. She was freed on $75,000 bond, CNN affiliate KTVT reported