It really depends on the school..normally after attending 2 sem, they will grant u cpt in ur field of study..it can be full time or part time..the maxm amount of time u can be  on cpt is 12 months, if more than 12 months of cpt is done, u won't get opt..

u can work offcampus on cpt..CPT is approved by ur college and more specifically by ur dept..DSO updates your I20 after u r approved for cpt..But , u need to have a job offer letter to be eligible for cpt..without job offer letter u can't get cpt..I worked on full time cpt for 2 full sem ..