The best time is when the new applications are released, which usually happens in July and August.
Also, it depends on what kind of help you need. If you're looking for help throughout the process, the earlier the better. However, if you're looking only for a sanity check on a particular essay or on a near-final draft of your essays, most of us usually have hourly retainer services where you can simply send it over a week or two before the submission deadline.
When it comes to the actual work (whether you use a consultant or not), expect to spend around 6 - 8 weeks on the applications, assuming you are applying to around 4 - 6 schools. The first two will usually take the most time, but beyond that, every subsequent application becomes substantially easier and quicker to complete.
Good luck
Alex
alex@mbaapply.com
www.mbaapply.com
http://mbaapply.blogspot.com