When to get Usability Reviews of your software done

Usability Review is an extremely important part of getting a software to market. By usability review, we mean that we want to ensure that the software we are building is being tested by end users before release. A software that is built on the basis of requirements given by Product Management and coded by the development team could be way off the mark in terms of usability unless there has been a systematic effort to ensure that some end-users have had time to play with the software and their interactions have been recorded.
Now, the best way to get people to give their feedback on a feature is to ask them to actually use the feature from within the software. This assumes that enough of the product schedule has happened that you have an application which is stable enough to get it reviewed by a end-user, and yet you need enough time in the schedule left that you can incorporate any changes suggested by the users. Sometimes what happens is that in the quest to get a better working software to show to end-users, the time period in which this usability testing is carried out is so late that it gets difficult to incorporate any feedback without impacting the schedule. The actual time period for this testing needs to be worked out pretty carefully.