At steel lintels above windows, how far should the edge of the through-wall flashing project beyond the edge of the lintel? I would think that the flashing does not need to extend beyond the outside edge of the lintel as any water reaching this location will just fall down to the windowsill below.

I generally recommend projecting the flashing out beyond the face of the brick, which may be 1 inch or so beyond the edge of the lintel. At the jambs, the lintel flashing will continue beyond the end of the lintel, which typically extends 4 to 8 inches into the wall beyond the window jambs. At this point, sealant must be placed between the underside of the flashing and the top of the brick below in order to prevent leakage. I typically use a metal edge strip or rigid metal flashing above lintels to create a straight, clean flashing edge along the entire length of the lintel. If the flashing does not project to or beyond the face of the masonry, it will not be possible to create a seal between the bottom of the flashing and the top of the masonry below. If the flashing at the window jambs is concealed, water reaching this location will flow around the toe of the lintel and down to the masonry at the jambs, contributing to leakage and deterioration problems.