How often do all the planets align?
When three or more planets appear 'to fall in line' we say 'they align'. When the planets appearing in a line are joined by straight lines, in order, the configuration will be a space polygon..
This space polygon connecting all the eight planets that orbit in different planes, and orbits of different sizes, would never appear to be aligned. The rare 5-planet alignment that we saw recently is due to the lines of the directions to the planets (at varying distances in AU unit) are very nearly coplanar. Near perfect alignment in a straight line is possible for the triad Moon, Earth and Sun. These events are called Total Lunar/Solar eclipses.