I was given my first real important project and asked how long it might take for me to complete it. I thought I could get it done in four months but I was scare

I was given my first real important project and asked how long it might take for me to complete it. I thought I could get it done in four months but I was scared. So I said six months. After the project was done, I was told that management didn't believe my six month estimate and put down nine months for completion in their timeline. I finished in three. Thinking I had three months to work on personal projects, good off, etc., I eventually gave in out of boredom and told them I was finished a few weeks later.