The PCT Applicant's Guide (National Phase) is the resource you are looking for. It contains a detailed guide on the WIPO Rules and Articles from the Applicant's perspective, and covers the time limits in detail, with pointers to information on exceptions for certain patent offices and explanations on how to address delays.
From the table linked via the excerpts below, the time limit for electing the United States is 30 months under PCT Article 22 and also 30 months under PCT Article 39(1).
Articles 2(xi), 22
3.001. What is the time limit for entering the national phase before a designated Office?
Subject to what is said below, no designated Office may normally process or examine the international application prior to the expiration of 30 months from the priority date and any fees due to a designated Office and any translation of the international application to be furnished to a designated Office will have to be paid and furnished, respectively, only by the expiration of that 30-month period (“priority date” means, where the international application contains a priority claim, the filing date of the application whose priority is claimed, and, where it does not contain such a claim, the filing date of the international application. Where the international application contains two or more claims, “priority date” means the filing date of the earliest application whose priority is claimed). In respect of certain designated Offices, the applicable time limit is 20 months, not 30 months because of the incompatibility, for the time being, of the modified PCT provision (PCT Article 22(1)) with the relevant national law; those Offices made a declaration of incompatibility which will remain in effect until it is withdrawn by the respective Offices. Furthermore, certain designated Offices have fixed time limits expiring even later than 30 months, or 20 months, as the case may be. For regular updates on these applicable time limits, refer to the relevant National Chapters; a cumulative table is also available from WIPO.
Articles 2(xi), 39(1), 64(2), Rule 54bis
3.002. What is the time limit for entering the national phase before an elected Office?
The time limit is normally 30 months from the priority date, the same time limit for entering the national phase as that which applies in the case of a designated Office which has not been elected (see paragraph 3.001). In respect of the designated Offices, for which the 20-month time limit applies (see paragraph 3.001), the time limit is 30 months from the priority date if the applicant files a demand for international preliminary examination prior to the expiration of 19 months from the priority date. The national law applied by each elected Office may fix a time limit which expires later than 30 months from the priority date. The National Chapter (Summary) relating to each elected Office in respect of which an extended time limit applies indicates the length of that time limit as does the table referred to in paragraph 3.001. It should be noted that some Offices apply different time limits depending on whether the applicant enters the national phase under Chapter I (Article 22) or Chapter II (Article 39(1)) (for details see the table referred to in paragraph 3.001). In those cases, it is recommended that applicants enter the national phase before elected Offices no later than 30 months from the priority date in order to ensure timely national phase entry.
Time limits applicable for entry into the national phase: Under PCT Article 22(3): 31 months from the priority date; Under PCT Article 39(1)(b): 31 months from the priority date.