If someone has lymphoma when would they start losing like so much weight they could tell?

Yes. I think you are asking 'if a DX of lymphoma has been made by an oncologist (cancer specialist), would weight loss start occurring, and if so, would it be so much weight loss that it was noticeable.' weight loss is not necessarily a part of lymphoma. If one is losing a good deal of weight from the lymphoma, it is not a good sign. If wt loss due to chemotherapy/nausea, different story. Ask the doc.
Variable... Sometimes unintentional weight loss occurs before a diagnosis of lymphoma is made, other times it does not. For hodgkin lymphoma, for example, unintentional weight loss of >10% puts you in a higher risk category requiring more intense treatment. For other lymphomas, it does change treatment. Another way to phrase things is, you can still have lymphoma with no change in weight.