There is no specific rule and generally ATC will issue the instruction to go around. However at some airports in the UK rules exist ( known as `land after` ) which allow, provided certain criteria regarding spacing and visibility are complied with, for the second aircraft to touch down with another aircraft still on the runway.
When it is foggy then a whole different set of rules apply. Greater spacing is needed to protect the ILS integrity and allow time for slower speed egress from the runway. Generally around 2min. spacing is applied but I am sure more knowledgeable ATC guys can speak about these rules.
Last edited by Meikleour; 8th Jul 2011 at 15:54.