Pretty simple, actually... As always, start by defining the relevant terms.
'Destructive' relationship = one that causes measurable distress, discomfort, and/or emotional, spiritual, financial, or physical damage to one or more of the persons involved.
You can compare the situation you're in against a hypothetical 'base-line' of not being in any relationship at all.
If you are in a 'Destructive Relationship', then you should get out ASAP. Most people tend to stick to their usual behavioral patterns rather than change their approach, in case the other person is not responding. If you have an abusive spouse, who likes to demean or physically abuse you when you disagree, then every time you disagree, the behavioral pattern will simply reinforce itself, becoming more and more prominent and intense.
Whatever kind of abuse you may be experiencing, chances are that the abuser will only get more aggressive over time - until they hit 'rock bottom,' that is... In some cases, that rock-bottom could end up being a dead body, possibly yours...
So getting out ASAP is a good and effective strategy, and the safest one. However, in some cases (for example, a long-term relationship, such as marriage, that is failing), some therapy or counseling may be a good recourse, before attempting to cut your losses.
Overall, weighing the potential benefit versus the potential harm of any given choice and basing your final decision on whichever is greater is always a good approach.