I think that the time limit has, historically, been more about pilots' privacy concerns than about technical constraints.
EASA already mandates 24 hours at least and the FAA is trying to match it, at least for new aircraft.
https://www.faa.gov/newsroom/media/25-Hr-CVR_NPRM_Issuance.pdf
EXCERPT:
"C. Privacy Concerns
The FAA acknowledges that pilot-focused organizations may have concerns regarding how the NTSB or the FAA would use the CVR data collected for investigative purposes. This issue previously arose when the FAA increased the CVR recording duration from 30 minutes to 2 hours. At that time, the FAA determined that the investigative need and benefit of this information outweighed these privacy concerns. The FAA maintains this stance. The proposed increase to a 25-hour CVR recording duration would further improve current investigative capabilities. It would also provide investigating bodies, such as the NTSB, with more complete context surrounding the accidents and incidents under investigation and support their safety analyses. Importantly, this proposed increase is designed to provide more context for any flight deck activity that might be pertinent to an investigation. Specifically, this increase expands the possible range of data available to investigators. This proposal does not alter or modify the existing processes for requesting or use of this data. Sections 91.609(g), 121.359(h), 121.227(f), and 135.151(c) specify that the information obtained from the CVR recording is to be used for investigation purposes and that the FAA will not use the CVR record in any civil penalty or certificate action. This proposal does not modify these regulations."