The test is whether the situation is getting worse with time and whether it presents a health or property risk. Sewage indoors is both, and needs attending now.
A slow-draining sink, an occasional gurgle or a smell is not an emergency — it is a strong signal to book a visit soon before it becomes one.
While waiting for an engineer, stop using water in the property, keep people and pets away from any contamination, and photograph everything if insurance may be involved.