Does work study from college can be a full time job ?

No. Work-Study jobs are limited to a maximum of 20 hrs/week, and total (gross) earnings are limited the amount stated in your financial aid package offer. So if, say, your Work-Study is limited to $5000, when you hit that amount, it's all over. Even if you have been working less than 20 hrs/wk.

Typically federal work-study program jobs are limited to 20 hours per week, except during vacations. 

sure it can.  the amount of studying and preparing one needs to do to do well in class can be a full time job.