I spent about £3500 in six months, which obviously isn't much at all!! But bear in mind back then the exchange rates were much better (2006/2007). I'd say allow £4-5k for six months nowadays. You can still do Asia really cheaply if you don't mind budgeting a lot. But then again if you spend an extra couple of quid the difference in quality is vast!
I.e you could spend £5.00 a night on a real dump, or £7 a night on a 3* decent room!