When should I start applying to biotech/pharma jobs (current senior in college)?

I’m majoring in Chemical Engineering and when I graduate, I’m looking for a Process Development type job at a biotech or pharma company in the Boston/Cambridge area. Our career office tends to funnel engineers into consulting/finance/etc and has limited resources for anything else, so I really have no idea what the job search timeline is like in this industry.

If I can’t start working until August 2019 (when my apt lease ends), when should I start applying to job listings? And when are there usually the most opportunities available?

Also, I’ve spent 3 summers interning at a clinical-stage biotech company in Cambridge, so I definitely plan on contacting my connections there as well as coworkers who’ve moved on to bigger pharma companies. But when should I start reaching out to them? And is it appropriate to do so on LinkedIn?

I’d really appreciate any guidance on this. Thank you in advance!