There is no hard threshold. As linked as a suggested duplicate to another question of yours, this answer cites the relevant statute. 18 U.S. Code § 1702:
Whoever takes any letter, postal card, or package out of any post office or any authorized depository for mail matter, or from any letter or mail carrier, or which has been in any post office or authorized depository, or in the custody of any letter or mail carrier, before it has been delivered to the person to whom it was directed, with design to obstruct the correspondence, or to pry into the business or secrets of another, or opens, secretes, embezzles, or destroys the same, shall be fined under this title or imprisoned not more than five years, or both.
As a preliminary point, 1702 does reach the act of a defendant "taking" mail that had been misdelivered into the defendant's own mailbox. Note the language: "Whoever takes any letter... which has been in any post office or authorized depository." And see United States v. Brown, 425 F.2d 1172 (9th Cir. 1970). In Brown, the defendant tried to argue that because the mail was put into his own mailbox, he didn't "take" it. The Court acknowledged that a predecessor to 1702 had that limitation, but said that the current version does not.
But as you can see, the statute does not prohibit merely keeping the misaddressed or undelivered mail. In the hypothetical you present, the prosecution would need to prove "design to obstruct" and "secreting" of the mail.1
These are evidentiary questions. See How do you prove a fact at issue in litigation? All relevent evidence would be considered. That likely includes evidence about the length of time you kept the mail or evidence of animus between you and previous occupants (all of which may be circumstantial evidence of intent or design), as well as testimonial evidence from the accused (who might try to provide another reason for keeping it).
We cannot tell you how a jury (or judge sitting as trier of fact) would weigh that evidence. All I can say is the longer you keep the mail without a plausible alternative explanation, the more likely the prosecution will be able to make out their burden. To be clear, the burden will remain on the prosecution to prove the elements of the offence beyond a reasonable doubt, so there would never be a true burden on the accused to present an excuse, but after a strong case from the prosecution that appears to make the case beyond a reasonable doubt, there may be a practical burden on the accused to introduce a reasonable doubt through competing evidence.
1. The parsing of the provision might on a quick read understood as only requiring "design to obstruct" or secreting of the mail (or one of the other listed prohibited actions), but the "or opens" is actually paired distantly with "takes". Parse it like this:
Whoever takes any letter ... (with design to obstruct the correspondence, or to pry into the secrets of another), or opens, secretes, embezzles, or destroys the same...
See e.g. United States v. Gaber, 745 F.2d 952 (5th Cir. 1984). This parsing is also evidence from the parallelism of the verb conjugations. "Takes," "opens," "secretes," "embezzles," and "destroys" are all conjugated, while "to obstruct" or "to pry into the secrets" are in the infinitive, connected with "with design."
I have picked out "design to obstruct" and "secretes" because on the facts of the hypothetical they seemed the most relevant. But on other facts, it might be a "design to pry into the secrets" and "opens."
