I think this is an unanswerable question, but a very common one. Defend oneself against what threat, exactly? People tend to think of self-defense skills kinda like having a fire extinguisher handy. If a fire happens, you grab the fire extinguisher and problem solved. Due to the wide range of threats one's safety, self-defense isn't so simple.
Even using the word 'tough' is precarious. I pose that 'smart' is far more likely to save one's butt from harm than 'tough'.
A few sober realities of self-defense:
- You can be peaceful and benevolent and still be in a violent encounter. That choice may be made by someone else.
- All your best efforts at self-protection (awareness, de-escalation, negotiation, etc.) may still fail.
- You can be the toughest guy in the world and still lose.
- You can do everything right, and still lose.
- There is no such thing as being invincible. The expectation of being so highly skilled or so incredibly tough that you will not lose a fight or attack is common among laypeople and martial artists, but it is a complete falsehood. The best you can do is train hard to improve your odds of surviving.
Which brings us back to your question. How much is well trained? My answer: more than I (or you) are doing. There is always more to go. The work is never done.
Lastly, martial arts skills are perishable. They fade with disuse. The good news is they return faster that it took to build them originally, but any physical skill can get rusty unless you stay in practice.