The simple answer is that there is no fixed limit or hard rule.
The essential question is was the process ‘fair’. That depends partly on the way in which consultation was undertaken and partly on how long it lasted.
Major public consultations normally allow 12 weeks. But with something like consultation on a proposal to become an academy, less would almost certainly be fine (say 6 weeks) depending on how widely consultation is needed, how consultees are communicated with, and so on. So, for example, if an individual letter was being sent directly to each person who could reasonably be expected to be consulted, then less time would be needed than if the information was simply (say) put on a web site. But bear in mind that just sending letters could itself be unfair – many people do not find that an accessible way to receive and give information. If information is provided and received in a number of ways: direct letters, on-line, public meetings (at different times of the day) then the process is much more likely to be ‘fair’ overall and a lesser timescale could thus be justified.
It is also important that consultees are provided with full information at the outset. If stuff is missing, and they ask for it during the consultation, it would generally then be necessary to extend the consultation – people need proper time from the point at which they have the information they need. So a 12 week consultation where the details were only trickled out by the consulter would probably be much less fair than a 6 week period where the consulter made everything clear at the beginning.
But the thing which most consultees complain about in my experience is that the consulter appears to have already made up their minds. That is plainly a problem where (for both academy conversions and free school set ups) the Academies Act 2010 allows the consultation to take place very late in the process, when the inertia behind the process may well make it unstoppable. So a claim that the consulter is openly-mindedly asking the question (as required by the Act) of whether to proceed stretches credibility. As my post explains, consultation should take place at a ‘formative stage’ (i.e. before minds are made up) and the responses to it must be ‘conscientiously taken into account’. It’s not obvious how that would be the case if the school is (as you put it) “being bullied by the DfE into voluntarily becoming an academy” (on one view your GB looks closed-minded against the conversion, on the other it is being pushed to one answer!). I suspect that is what partly inspired new powers in the Education Act 2011 which will allow the Secretary of State to order discontinuance of a maintained school with some new third party being invited to undertake the consultation with a view to then running the school as an academy – see this post.