How I've seen Skallagrim's take on autohits via lack of posting; 

Day 1; Makes an attack. 

Three/Seven days later, no response, connects attack (insta-kill if it was of the "I try to chop your head off with my sword"-nature, if not;

Day 3/Day 7; Makes another attack

Three/Seven days later; Instakill.

If the first period in this example was posted as a non-attack, then the second period, which should be an attack, would still be followed up by an insta-kill. In basic, the formula is; 
Post - Post - Instakill.