How long would it take to ride from Brisbane to Perth?

It will take much longer than 47 days - you can usually average 100km a day on a bike on a long trek.THe distance is 4300km - So will relate to 45 days. The costs for that many nights accommodation, food etc (there would be about a week between food and water stops along the nullabour - you would need to carry 10-20kg of water for that trek). It is not realistic to do without a support vehicle or two.