You should do a qualitative fit test of your respirator:

A qualitative fit test of a respirator is essential to ensure that the equipment provides an adequate seal to the wearer's face, thus preventing exposure to hazardous substances. The choice indicating that a fit test should be done if there is a significant change in weight is based on the principle that such changes can affect the fit of the respirator. Weight loss or gain can alter the contours of the face, potentially compromising the seal. Therefore, to maintain safety and effectiveness, it is critical to reevaluate the fit of the respirator whenever there is a significant change in the person's weight.

The other options suggest intervals that are not responsive to individual changes in fit. For instance, testing every five years or semiannually does not take into account any personal changes that might occur in that time frame. Testing only after using the respirator for the first time also misses the proactive approach necessary for ongoing safety adjustments. Qualitative fit testing should be conducted whenever there's a chance the fit has been altered to ensure continued protection.