It depends on the situation and the person. This question is difficult to answer in the same way it's hard to say how much sun causes a sunburn: the amount varies from person to person. What one person can tolerate with little or no effect may cause symptoms in another individual.
The long-term presence of indoor mold may eventually become unhealthy for anyone. Those with special health concerns should consult a medical doctor if they feel their health is affected by indoor mold. The following types of people may be affected sooner and more severely than others: