Well I was away for 18months, 12 months travelling and six months working in Australia. Disregarding the six months I spent working I would estimate I spent just over £1k a month. If you spend more time in Asia and less time in Oz then your monthly spend would be much cheaper. I was spending over £1k a month in Oz, trying to live as frugally as possible, ie always making my own meals and hardly ever going out drinking.
I wish I had spent less time travelling around Australia (I spent about four months in Australia) and more time in SE Asia (I spent around four months there). New Zealand is fantastic and the Yasawa Islands in Fiji are beautiful.
I would say £10k between you could last anything between four and nine months roughly, depending upon where you go or what you do.
Hope this helps