I completely understand your frustration. Cooking traditions passed down from family are wonderful, and it can feel unnecessary when modern recipes add what seem like tedious extra steps.
To give you a straightforward answer: the general rule of thumb is to let the batter rest for 1 to 2 hours in the refrigerator, though a brief 15 to 30 minutes on the counter will still give you a noticeable improvement. If you prefer to make your batter ahead of time, leaving it overnight in the fridge works wonderfully too.
Your grandmother was likely able to skip this because her recipe used lower-protein pastry flour or she had developed the muscle memory to mix the batter very gently without activating the gluten. When batter rests, two simple things happen: the flour grains fully absorb the liquid, and the gluten strands created during whisking relax. If you skip resting altogether, your crepes might still taste good, but they can end up slightly rubbery, tear more easily in the pan, or develop small holes from trapped air bubbles.
One practical tip to keep in mind: crepe batter naturally thickens as it sits. When you are ready to cook, give it a gentle stir. If it seems thicker than heavy cream, simply whisk in an extra tablespoon of milk or water to bring it back to the right consistency.
