Tax Professionals, estimated quarterly taxes...Errr...?

How much does someone have to make in a certain year or expect to make in a certain year to be required to pay estimated quarterly taxes? I started this job in September of last year. Before that, I worked at another job from January to June and was unemployed for the whole summer. I had a no tax liability for last year. This year, I may only be employed until June. My boss isn't sure yet. If I don't work in the summer, I may or may not be back there in September. If I don't work in the summer, I would be making LESS than $7000 for this year. So, am I required to file estimated taxes for January to March of this year?