Most people wait too long. The usual reason is not denial but uncertainty about whether what they have noticed is enough to bother anyone with.
Here is a way to think about it.
Book something in the next few weeks if any of these apply.
you do not need to be certain before going. Working out whether there is a problem is the appointment's job, not yours.
the calculation is different when it is not your own memory.
They may not have noticed, or may have noticed and be frightened. How to talk about memory concerns covers raising it, and preparing for a doctor's visit covers what to bring once they agree.
If there is an immediate safety risk, driving, medication or finances, that changes things and belongs with their doctor now rather than when they are ready.