The deadline is short, it runs from the loss and not from the adjuster's visit, and it can be extended only in writing. Maine's codified standard fire policy, a version of the wording many states enacted, requires that "within sixty days after the loss, unless such time is extended in writing by this Company, the insured shall render to this Company a proof of loss, signed and sworn to by the insured," and then lists the same categories of information the flood policy uses: the time and origin of the loss, the interests of the insured and of all others in the property, the actual cash value of each item and the amount of loss claimed, all encumbrances, all other contracts of insurance, and any change in title, use, occupation, location, possession or exposure since the policy was issued. The Standard Flood Insurance Policy sets the same 60 days and closes the obvious escape route: the adjuster "may furnish you with a proof of loss form, and she or he may help you complete it. However, this is a matter of courtesy only, and you must still send us a proof of loss within 60 days after the loss even if the adjuster does not furnish the form or help you complete it." Under that same policy the insurer may, at its option, accept the adjuster's signed and sworn report in place of the insured's proof of loss, which is how many routine claims are settled without one ever being filed by the policyholder. That is an option the insurer holds, not a waiver the policyholder can assume.
What the document does, in three separate ways. First, it fixes the insured's own number. The flood policy tells the insured to use their own judgment on the amount and to justify it, which means the figure is a claim the insured is prepared to swear to rather than a repetition of the adjuster's estimate. Second, it starts the payment clock. The standard fire wording makes the amount "payable sixty days after proof of loss ... is received by this Company and ascertainment of the loss is made either by agreement between the insured and this Company expressed in writing or by the filing with this Company of an award" under the policy's appraisal clause; the flood policy is written the same way, with a 90-day alternative when the adjuster's report is used instead. Third, it triggers the insurer's own options. Under the standard fire policy the insurer has 30 days after receiving the proof of loss to give notice that it intends to repair, rebuild or replace rather than pay, and the flood policy sets the same 30 days for its equivalent election.
Getting it wrong is one of the few claim mistakes that can end a valid claim. The standard suit clause says no action on the policy is sustainable "unless all the requirements of this policy shall have been complied with," and then adds its own limitation period, two years after inception of the loss in Maine's wording and one year after written denial in the flood policy. A proof of loss that was never filed is a requirement that was never met. The practical protections run the other way too: the flood policy permits an amended proof of loss, provided it is filed within 60 days of the date of the loss, and a mortgagee who would otherwise lose out gets its own backstop, since a mortgagee that submits a sworn proof of loss within 60 days after being told the insured failed to do so keeps its claim even where the insured's claim is denied. Standard fire wording gives the mortgagee a similar 60-day window.
Two claim-practice standards bear directly on the document. The NAIC Unfair Claims Settlement Practices Act, the model law most states have adopted in some form, makes it an unfair claims practice to unreasonably delay investigation or payment "by requiring both a formal proof of loss form and subsequent verification that would result in duplication of information and verification appearing in the formal proof of loss form," and to fail "to provide forms necessary to present claims within fifteen (15) calendar days of a request with reasonable explanations regarding their use." So an insurer is expected to supply the form promptly when asked, and to avoid demanding the same substantiation twice. A model act is not itself law anywhere; what binds is each state's own adoption of it, which is where a policyholder with a dispute about claim handling should look.