It is my understanding that he was charged under 18 USC § 1001 which gives a maximum sentence of 5 years under most circumstances. This is the absolute maximum; the judge is not allowed to give more than the law allows.
Here are the federal sentencing guidelines. If you look in the guidelines, you'll see that the base offense level for this crime is 14. If there was a "substantial" interference with justice, the offense is increased by 3 levels. My guess is that they won't find this to be the case. There are several other adjustments that likely don't apply.
Under the "Adjustments" section of the guidelines, it says:
(a) If the defendant clearly demonstrates acceptance of responsibility
for his offense, decrease the offense level by 2 levels.
(b) If the defendant qualifies for a decrease under subsection (a),
the offense level determined prior to the operation of subsection (a)
is level 16 or greater, and upon motion of the government stating that
the defendant has assisted authorities in the investigation or
prosecution of his own misconduct by timely notifying authorities of
his intention to enter a plea of guilty, thereby permitting the
government to avoid preparing for trial and permitting the government
and the court to allocate their resources efficiently, decrease the
offense level by 1 additional level.
It seems that the first one applies. The second one cannot apply in this particular case since the offense level was not 16 or greater. So the offense level would be reduced from 14 to 12.
If you look at the Sentencing Table, you can see that an offense level of 12 has a recommended sentence of 10-16 months for someone with no prior criminal history. Also, this is in Zone C, meaning:
the minimum term may be satisfied by... a sentence of imprisonment that
includes a term of supervised release with a condition that substitutes community confinement or home detention according to the schedule in subsection (e), provided that at least one-half of the minimum term is satisfied by imprisonment.
so it seems he would go to prison for at least half of that. But if the offense level was dropped just one more level, he would be in zone B, where probation with home confinement would be an acceptable sentence.
But there's one more thing to consider: He's cooperating with investigations into other people. This allows a departure from the ordinary guidelines.
Upon motion of the government stating that the defendant has provided
substantial assistance in the investigation or prosecution of another
person who has committed an offense, the court may depart from the
(a) The appropriate reduction shall be determined by the court for reasons stated that may include, but are not limited to, consideration of the following:
(1) the court’s evaluation of the significance and usefulness of the
defendant’s assistance, taking into consideration the government’s
evaluation of the assistance rendered;
(2) the truthfulness, completeness, and reliability of any information
or testimony provided by the defendant;
(3) the nature and extent of the defendant’s assistance;
(4) any injury suffered, or any danger or risk of injury to the defendant or his family resulting from his assistance;
(5) the timeliness of the defendant’s assistance.
I have no idea where the Guardian got 6 months from. It's certainly possible that's part of the plea deal (although nothing says the judge has to impose the sentence that the prosecutor recommends as part of the deal), and maybe there are adjustments I'm not taking into account that would reduce the offense level further.
I just now found a copy of the plea agreement. It specifies that the parties agree he will be sentenced under guideline §2B1.1(a)(2). This ordinarily is for offenses such as theft, property damage, forgery, or fraud. As far as I can tell, the government is just using this to get the sentence down, because there's no real relation to the crime he's accused of.
Under that guideline, the base offense level is 6. The two level decrease for accepting responsibility puts it at an offense level of 4, with a possibility of a downward departure for cooperation. Regardless of whether or not there's a downward departure, this would have a 0-6 month sentence recommendation, which is the lowest available in the guidelines. This is also in Zone A of the sentencing table, meaning a sentence of just probation with no imprisonment or home confinement is possible.