You can start right-away if you want. Just don't start a full-fledged preparation.
Keep brushing your Math and Verbal skills from time to time. Give about 30 minutes a day to refreshing concepts so that you don't forget them over time. Once you have graduated from college and started working, I am sure that you can give at least 30 minutes or an hour maximum for GMAT Math and Verbal.
Once you have achieved sufficient work experience (at least two years), start your GMAT preparation in earnest. You should give at least two-three months for thorough preparation. Don't quit work during this time though, as it may be counted as a negative perception during your admission.
No time is too early. It all depends on how much time you can give. Just don't burn yourself out during studying.