Is there a deadline for my mother to make known her choice regarding the inheritance of my father (her husband)?

No, unless expressly requested. The law provides no time limit for the surviving spouse to choose between the available inheritance options (in the presence of joint children: ¼ full ownership or full usufruct).
However, an heir may ask the surviving spouse in writing to exercise their option. If the spouse has not chosen within three months of the death, they are deemed to have opted for full usufruct of the estate (Art. 758-3 Civil Code). The same option applies if the spouse dies without having chosen (Art. 758-4 Civil Code).