Judging by the length of the passage I'd say a week either side of the mid-point at least. I was up at Carrowkeel Cairn G nearly a week and a half after summer solstice sunset last year and it was still possible to see the sun set through the roofbox. The passage there is a probably a little shorter than MOTH but the angle at Tara is odd. I'm sure the same applies no matter what the angle though the sun moves quicker on the horizon at this time of year.