I don’t believe one can set an “ideal time”, with that being said, if a proper assessment is conducted, and a solid integrated project plan is put in place, you can build an aggressive scenario in which you choose to prioritize the people element. Transparency and engagement then become key deliverables of your integration project. Let’s say depending on the size and nature of your organization, you would aim to have all staff onboarded and/or released by a certain date, which is just another way of saying, you will provide staff with answers as soon as possible, and you will communicate your plans also as soon as possible. The ideal date in my opinion is “as soon as possible”!
Size of the company matters from the point of view that decisions regarding staff involve mapping exercises, assessment of skill sets and business needs, etc. The larger the company, the longer this process may take, depending of course, on how many resources you have dedicated to these efforts.