Accounts refresh constantly which means they will receive new transactions the moment they are posted to your account. There aren't intervals where Monarch needs to go out and check for new transactions once a day or every few hours since everything happens in real-time.
However, many financial companies don't post new information in real-time. They will often batch up new transactions and post them once or twice a day - sometimes waiting until the end of the day.
You can manually force a refresh which will go out and check all transactions and compare them to what is in Monarch. This takes longer than just "listening" for any new transactions but it can be a good first step to troubleshooting if you notice missing transactions.
To refresh, click the blue arrows shown below!