Upload the MKV and open the options. Frame Extraction has two modes: Single Frame saves one TIFF from the second you enter (for example 1, 5 or 10.5), and All Frames saves every frame of the video as a TIFF image. There is no every-Nth-frame or one-per-second mode.
It depends on the size and length of the file. Small documents and images are usually ready quickly. Long videos take the longest, because most video conversions re-encode every frame.