As far as you are comfortable. It is a formula of time + money = satisfied hobbyist. Since all three of those are subjective variables, and life tends to happen to you to blow up your carefully laid plans, it's all a wash.

However, doing a "full restoration" on a beater and bringing it back to a cosmetically appealing game that plays snappy and right is a really fulfilling experience. After you did what was needed, then did what was not really needed but it made a significant improvement, then did what wasn't needed at all but you couldn't stop yourself, then had to do what was left because everything else you did makes what you didn't do stick out like a sore thumb...it's a great trip to go on.