I've got a HM coming up Sunday week and it's most definitely a target race. My version of a taper is to just really cut down the volume during the week leading up to the race, so effectively this evening's long tempo run was the last full-on quality session. Sunday's long run may be reduced slightly, I've been in the habit of knocking out 16 miles most weekends but may do 12 - 14. I'll do a cut-down interval session on Tuesday - not sure what we've got planned but maybe 4 x 800 instead of 8? - and a short tempo run on Thursday; it just happens that there's a club 5k race that I'll be running in but I'll happily run it around target HM pace given the short distance.
I think there are two elements to the taper, one being recovery of your muscles from hard training and the other being topped up stores of glycogen. I don't think you need anything like the taper as for a marathon because you're coming down from a smaller peak of volume, and you don't need to be packed to the hilt with glycogen stores for 13.1 miles. So a week to get over any really hard sessions seems long enough to me.