Your memory is too good. The supplementary information accompanying the proposed regulation contained this observation:
Currently, the IRS indicates that the issuance of an employer identification number can take up to five weeks. This length of time, coupled with when the person applied for the employer identification number, should be considered by the bank in determining the reasonable period of time within which the person should provide its employer identification number to the bank.
The analysis (focused on EINs, not TINs in general) was not repeated when the final regulation was issued.
More to the point, IRS Form W-9 has historically indicated that it can be used as an "awaiting TIN" certificate for 60 days from the date of its execution. There is no reason to impose a time frame on the IRS that is shorter than the IRS itself suggests is a possibility. The banks that provide for this exception in their CIP generally use 60 days as the time frame. I suggest that anything longer is simply not reasonable and that anything shorter will create unnecessary problems.
(I'm not reaching here, a footnote in the supplementary information accompanying the final CIP rule makes a specific reference to the IRS' "awaiting TIN" concepts.)