Animate the clip’s opacity in invideo Editor. For a fade in, begin with an opacity keyframe at zero, then add a later keyframe at the visibility you want. Reverse that progression for a fade out.
Move the keyframes farther apart for a longer fade or closer together for a shorter one. Preview the layer together with the footage underneath it: the result depends on both layers. Check that a title or graphic stays fully visible long enough to read before it fades away.