Our son has never been a brilliant sleeper BUT things were starting to improve - regularly doing 6 hour blocks and even a couple of 8 hours. But for the last couple of days he's been a grouch bag, clingy and whinging instead of his usual chirpy, smiling self. He's reverted back to waking every 30 mins and is utterly inconsolable. Had a look/feel earlier and sure enough, the first of his molars is on its way. Poor little sausage. Any ideas how long this might last?