Accounts will refresh in real time which means they will receive new transactions as soon as your financial company makes them available to apps like Monarch. There isn't any predefined time interval for when Monarch goes out to "check for new transactions".
However, many financial companies don't post transactions in real time. They will often batch up new transactions and post them once a day - usually at the end of the day. So for that reason Monarch will proactively go out and check for new transactions to catch any that the financial company hasn't told Monarch about yet.
You can manually force this full refresh account process by clicking the cycle icon in the connection status summary card. This process 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, go to the account in question and click the blue arrows shown below!