From what I understand 12 Hr DXM
medicine lasts twice as long as the regular stuff because the 12 Hr stuff is bonded with an edible plastic and takes longer to break down in the digestive system for this reason. Therefore, SWIY should be able to consume twice as much as would normally be consumed with relatively the same effect but SWIY should count on the effect lasting doubly long considering it takes about double the amount of time to break down in the human digestive system, unless aided by an outside source such as alcohol
or any other acceptionally corrosive substance. In that case I cannot say. What I know for sure is that Robitussin extracted DXM makes for a 4-6 hour trip in most humans and Delsum which contains the pre-mentioned time release chemical is intended to last 12 hours, trip or no.