The basic character simplication is about 10%. More if combined the simplified form with other characters. However, if I were to take your question literally, the percentage would be slightly higher because there are some characters which are simplified a long time ago which most Chinese think is not simplified. For example, 灋. The simplified form is 法! The original encoded meaning is that justice must be fluid like water. The 廌 is a mythical monster that will pounce and eat up the guilty. A statue of this beast is always carried out in a Chinese court, or Yamen when a judgement is to take place. Or characters such as 烟 for 煙 is not really a simplification but a variant form used in Chinese calligraphy. Since it contains fewer strokes, it is the preferred form for simplication. There are other criteria, some good and some bad. It is best to know both forms.

Why do you want to know that percentage? 简体字和繁体字，你选一种喜欢的学习就好了。 我没法告诉你繁体字简化的百分比，只能告诉你使用简体字的人比使用繁体字的人多得多。

I can't give you a percentage. But I think almost every single character has been simplified

most chinese people use the characters which has been simplified.

I learnt both simplified / traditional Chinese, the total percentage of simplification of the character set is small, as Jeff said. However, the simplification was focus on "most commonly used characters", so in real daily life usage, the percentage is much higher, I think it is more than 30%, it depends on the context.