One almost wants to say in the early days when things are going well and both individuals are more open and generous towards each other, and wanting to be loving towards each other. It’s like saying, let's get down the basic foundation of good habits in place to help us work through any challenges we will inevitably encounter.
However, understandably, no-one is inclined to use therapy in this phase. Unfortunately, what’s more likely to happen is that people often leave it too late by which time problematic patterns and dynamics have become entrenched and harder to shift.
With that in mind, the ideal time would be when one or the other in the couple sees patterns that create problems and disrupts the relationship - be that in mood, behaviour, personality, etc. They might have tried to address it between themselves, and find themselves going round in circles, unable to resolve things or move forwards.
If one person within the relationship finds themselves starting to feel unhappy, resentful, or smaller (as a person) than they used to be, this would be the time to recognise that some external support might be required.