If the evaluation is finished in a couple of weeks or a month, be suspicious. It doesn't necessarily mean the evaluation was rushed or done improperly, but it's at the extreme low end of the time-scale for these things. It may mean that certain procedures were 'streamlined' or skipped, and you should inquire as to whether or not all the relevant people were contacted, etc.
Conversely, if the evaluation is taking a long time (5 or 6 months or more), this may also be reason for concern. It may be that the evaluator is lethargic in his/her duties, unmotivated, or incompetent. It may also mean that they're 'milking' you financially for all they can get. It could mean that the evaluator has been unable to contact the relevant parties, or that the opposing attorney (or his client) is deliberately stalling the evaluation, dragging it out and making it take an abnormally long time.
When this happens, the judge may ignore the evaluator's findings and decide to "preserve the status quo", making no changes in the custody arrangements. This situation is most often seen when one parent has full 'temporary' custody and wants it to remain that way. By prolonging the evaluation unreasonably, they set the stage for the judge to rule that, because of the long elapsed time, the child has an 'established custodial environment' and no change will be made for the sake of his/her stability. Attorneys know this and may attempt to use it to benefit their client. This is another reason to vigorously insist on some form of shared parenting during the separation and/or evaluation period.