In response to my own question, I've determined a couple solutions.
For iPhone's, the user can access their 'general settings,' and then locate the email, contacts, calendar' option.
The mail settings are on the iPhone not on EMM – you have a choice of:
On Androids, the maximum is 1 month through Secure Container settings > sync settings.
However, users (for both iOS and Android) can search their entire mailbox no matter what sync time they choose.