A 2016 survey featured in an ABC news report:
"in a survey of 8,140 people conducted by Western Sydney University . . . of the respondents, 64 per cent said they believed the Australian flag should change, while 36 per cent said it should remain the same."
http://mobile.abc.net.au/news/2016-0...survey/7113992
Surveys are somewhat subjective but anyway this shows that many do want change.
An appropriate time to change will be when we become a republic. An appropriate time to become a republic is when there is a change of monarch - if it does continue after the Queen. I suppose this depends on how well Prince William is groomed for the position - I can't think anyone else would be acceptable.
P.S. I've seen flag designs that include a boomerang - looks to corny for my liking.