There are 2 separate issues here, data retention and data access. The first is governed by the value of the information and the legal or regulatory issue surrounding that data. For example in Europe personal data may only be retained as long as it is needed, tax data needs to be kept for X number of years etc. An evaluation of what you hold will enable you to draw up a retention schedule.
The next part of the problem is data access. No business data should be stored on local drives or in e-mail. Use either shared folders or a collaboration tool e.g. Sharepoint. This will also ensure that all of your business information is included in you backup routine.
Not forgetting of course that e-mail forms part of the company records and that should be retained for as long as required by your local regulatory environment demands.
Unfortunately there is no simple answer to the question `How long do I have to keep this information?.