You can choose how often you would like to fetch data from your Exchange Account Connections.
Head over to Settings -> General (or click here).
Click on "Sync Exchange Connections".
Select the interval you want the exchange connections to sync automatically. The recommended frequency is 30 mins or at a lower rate to minimize the chances of you getting rate limited by the exchange.
PRO TIP: If you get rate-limited due to the frequency at which your connections refresh, please lower the rate to hourly or even 6 hours. You can learn more about being rate-limited here.
PRO TIP 2: Some exchanges might not successfully refresh in the background due to it taking longer than 30 seconds to complete. This is due to the way the OS prioritizes certain app processes. To get around this, please open the app and wait for the sync to complete.