Soon! The sooner the better. In fact, you could even lay the groundwork ahead of time as a sophomore or a junior.
Your school should have career fairs or functions where businesses send representatives to scout talent at your school and exchange cards. Go to those. The whole purpose is that companies want to higher college grads -- they're doing the same thing you are. They want to make connections early so they can plan hires into their calendar.
If you don't like the school-wide events, talk to your academic advisor or a counselor within your department. Businesses usually have tie-ins with certain departments--sometimes it's for research, sometimes as a pipeline for internships, but one purpose is so they can establish a link to a future labor pool. So you have options.
You can prepare yourself early, too. Build a resumé and a cover letter for the jobs you want (and tailor them to specific companies, too; you don't have to create a cookie-cutter resumé--make multiples, go ahead and highlight specific skills that match specific companies). You can find templates online, but if your school offers a workshop on how to write one, go to that. Those two things are your "handshake" with your prospective employer and will determine if they grant you an interview or not. Start those now of you haven't.
But start early. Go to the fairs, talk to your advisors, and just open yourself up to job offers. You may be surprised what jobs are out there--you may not know that job even existed, but it may be the one for you.
Good luck! Hope it helps.