29 CFR §2560.503-1(f)(2)(iii)(B)

(B) Post-service claims.

In the case of a post-service claim, the plan administrator shall notify the claimant, in accordance with paragraph (g) of this section, of the plan’s adverse benefit determination within a reasonable period of time, but not later than 30 days after receipt of the claim. This period may be extended one time by the plan for up to 15 days, provided that the plan administrator both determines that such an extension is necessary due to matters beyond the control of the plan and notifies the claimant, prior to the expiration of the initial 30-day period, of the circumstances requiring the extension of time and the date by which the plan expects to render a decision. If such an extension is necessary due to a failure of the claimant to submit the information necessary to decide the claim, the notice of extension shall specifically describe the required information, and the claimant shall be afforded at least 45 days from receipt of the notice within which to provide the specified information. (Emphasis added.)