The recent ruling of Jennings v. Rodriguez gives some clues. Rodriguez was detained for deportation, and sued, alleging entitlement to a bond hearing. His legal team constructed a theory that there was a 6 month limit on detentions. The lower court agreed, and (as reported in the SCOTUS ruling)
Relying heavily on the canon of constitutional avoidance, the Court of
Appeals construed §§1225(b) and 1226(c) as imposing an implicit
6-month time limit on an alien’s detention under these sections. Id.,
at 1079, 1082. After that point, the Court of Appeals held, the
Government may continue to detain the alien only under the authority
of §1226(a).
That principle is relevant when a statute that is found to have multiple readings. At stake are the provisions of 8 USC 1225, where
respondents argue—and the Court of Appeals held—that those provisions
nevertheless can be construed to contain implicit limitations on the
length of detention
specifically an implicit 6-month limit on the length of detention, and "urge[d] this Court to use that canon to read a 'six-month reasonableness limitation' into §1225(b)". SCOTUS resoundingly rejected that interpretation. It stated that
§1226(c) does not on its face limit the length of the detention it
authorizes. In fact, by allowing aliens to be released “only if” the
Attorney General decides that certain conditions are met, §1226(c)
reinforces the conclusion that aliens detained under its authority are
not entitled to be released under any circumstances other than those
expressly recognized by the statute.
And together with §1226(a), §1226(c) makes clear that detention of
aliens within its scope must continue “pending a decision on whether
the alien is to be removed from the United States.” §1226(a).
This is not quite license for indefinite detention, but it indicates that non-statutory limitation on the duration of a detention doesn't arise from the detention being unreasonable.
In a related earlier case, Zadvydas v. Davis, 533 U. S. 678 the court held that a certain statute
implicitly limits an alien's detention to a period reasonably
necessary to bring about that alien's removal from the United States,
and does not permit indefinite detention
Duration, per se, is not the limiting feature, rather, it is "however long is reasonably necessary to remove the alien" (in the instant case, Z was essentially stateless and no country would take him, so detention is "potentially permanent").
The court also held that
The application of the 'reasonable time' limitation is subject to
federal-court review
and under such review,
the court must ask whether the detention exceeds a period reasonably
necessary to secure removal. It should measure reasonableness
primarily in terms of the statute's purpose of assuring the alien's
presence at the moment of removal.
In that version of SCOTUS, they found that
It is unlikely that Congress believed that all reasonably foreseeable
removals could be accomplished in 90 days, but there is reason to
believe that it doubted the constitutionality of more than six months'
detention. Thus, for the sake of uniform administration in the federal
courts, six months is the appropriate period. After the 6-month
period, once an alien provides good reason to believe that there is no
significant likelihood of removal in the reasonably foreseeable
future, the Government must furnish evidence sufficient to rebut that
showing.
A third case, Clark v. Martinez, 543 U.S. 371 reiterates that "the presumptive period during which an alien’s detention is reasonably necessary to effectuate removal is six months". However, the courts are not looking at the same statutory provision in these three cases. (I think the real reason for the difference is the different makeup of the court).
To summarize: it is an open question what constitutional limits on duration of detention exist. There is an earlier hint that 6 months might be a limit, and a later rejection of that hint (it just a hint, after all). An indefinite detention would, as stated in Zadvydas, raise serious constitutional issues, but as the most recent ruling finds, the limit might be in terms of satisfying a condition, not a period of time.
