I would start about 90 days depending on the area and the school you are attending. If it is a tight rental market there you may even go out to 120 days. Craigslist is good but be careful when contacting owners. Meet in public places.
Contacting a Realtor is a good choice because the Realtor will do the footwork for you and it's a safer bet. Depending on the rent it may cost slightly more though.
If you are going into the Greater New Haven Area I can help you out if needed.
Carbutti and Co. Realtors
Let me know if you have any additional questions, I'm always happy to help.
Citylights Realty Group