That's not true. No mention of 48h anywhere. Only place i found it is here: https://www.service-public.fr/particuliers/vosdroits/F35254
"Le préfet rend sa décision dans un délai de 48 heures, à partir de la réception de la demande." which roughly translate to "The prefect renders his decision within 48 hours of receiving the request."
We have a family house that was squatted on the coast during last winter. We asked the squatters to leave (while redacting a letter to the prefect) and they too believed the "48 hour" stuff. The police did not however, and made them leave quite quickly (less than a week).