Dinner at a ryokan, a traditional Japanese inn, does not work quite like ordering at a restaurant. Meals are planned around the number of guests, and ingredients may be purchased and prepared well before dinner. If you have an allergy or dietary restriction, the safest approach is to mention it when you make the reservation.
The earlier the inn knows, the easier it is for the staff to consider what they can do. Even then, advance notice does not mean every request can be accommodated. A ryokan may be able to leave out one ingredient but not redesign an entire meal. Smaller inns may also have limited kitchen space, staff, ingredients, or cooking methods available.
Severe allergies require even more care. If avoiding the ingredient itself is not enough, and shared oil, cookware, or utensils are also a concern, ask the ryokan about this before booking. This is a safety issue, so it is better to know exactly what the inn can and cannot handle in advance. If the answer is that they cannot meet your needs, choosing another place may be the safer option.
When guests explain their needs early and clearly, the staff may still try to help within what is realistically possible. That effort should not be treated as something the guest is automatically entitled to. If the inn is trying to accommodate you, it is reasonable for the guest to do their part too: give notice early, explain the restriction clearly, and avoid pushing for changes the kitchen cannot make.
The situation feels very different when a major request is made only after arrival, even though it could have been mentioned earlier. Expecting the kitchen to make large last-minute changes and assuming the staff should simply manage somehow can easily create friction.
There is also nothing unfriendly about a ryokan saying clearly, before your stay, “We can do this much, but we cannot go further.” That information lets the guest decide whether the arrangement is acceptable or whether another inn would be a better choice.
There is no single number of days that guarantees a meal change will be possible. Once you know you need an adjustment, tell the ryokan as early as you can and ask what they are actually able to provide.