Depends on how many hours one's prepared to put in at a go, but without going lunatic, say several hours a day, it took the wife 3 or 4 days to build one of these . . .
That's 3104 piece Imperial Star Destroyer (#10030), so I would guess the 3417 piece Death Star II (10143) would closer to a full 4 days and the 5195 piece Millennium Falcon (#10179) perhaps 6 or 7.