Even though there is no specific time-limit prescribed to present proof of export to jurisdictional tax-officer, same is expected to be presented within a reasonable time from date of export in normal course. This is considering the wordings used in above-reproduced clause at serial No. 1 (ix) (i.e. when goods have been exported).
Serial No. 2 of notification 40/2017 (CGST Rate) needs to be kept in mind i.e. The registered supplier shall not be eligible for the above mentioned exemption if the registered recipient fails to export the said goods within a period of ninety days from the date of issue of tax invoice.
Considering above, so long as subject goods are actually exported within prescribed period of 90 days and have supporting proofs as listed in clause at serial No. 1 (ix) thereof, mere delay in presenting those proofs to jurisdictional tax-office can NOT lead to denial of exemption given in notification 40/2017 (CGST Rate).
On practical note, it is in interest of supplier claiming exemption to avoid undue delay from recipient in presentation of proofs as mandated in serial No. 1 (ix).
These are ex facie views of mine, the same should not be construed as professional advice / suggestion.