It depends on how commercially significant the brand is and what the cost of missed infringement would be.
For a brand that drives meaningful revenue and whose identity would be damaged by a copycat, the cost of a monitoring service is typically modest relative to the value it protects. The most financially significant scenario it prevents is a confusingly similar mark being registered in your class and jurisdiction; challenging a registered mark is considerably more expensive than opposing an application during the original publication period.
For a small brand with limited commercial activity, self-monitoring through periodic database searches may be sufficient. The key is that some form of monitoring happens consistently, regardless of whether it is manual or automated.