GK that's a deep question. Having spent half my working life with women escaping domestic abuse, I think it varies from individual to individual and could be gradual, awakening or epiphany. Often a combination -gradual realisation that things are not right, leading to a change in attitude, followed by a lightbulb moment, triggered by a trivial or extreme happening but can only occur when someone is in the right frame of mind to receive the enlightenment.
As to depression it was explained to me that seritonin is the key. When you are happy you produce seritonin - seritonin makes you feel good - you feel happy so you produce more. When you are miserable (because of constant rain, stress, the flu, a bereavment) you produce less seritonin - you feel more miserable - you produce even less. Usually something breaks this vicious circle - a sunny day, a Grandchild's hug, a new hairdo introduces a little chink of light - some seritonin is produced, you feel better, produce more, feel happier, are more receptive to . Following this model the change would be incremental or cumulative.
[Clinical depression occurs when there is a problem with the receptors in the brain which fail to pick up the seritonin message. Hence Selective Seritonin Re-uptake Inhibitors (SSRIs) which are designed to stop available seritonin being reabsorbed into the system leaving it still around for longer, giving faulty receptors more chance to pick it up.]
Hope Jeni does not think I am supplying duff information here 