You can start looking whenever. I suggest looking around as early as possible, figuring out what you want, your budget, etc. What is nice is that while financial responsibility is still key, the loans you will need to take out will very much help you find a good place to live. In medical school, the they'll give you a silly amount of money b/c they know they're getting it back x2 in the future lol.
If it's Philly, it's usually center city or somewhere along or close to the PATCO line. You'll need access to it if you want to commute w/ it. 99% of Philly ppl use it daily.
If it's NJ, Haddon is lovely. Sleepy. Plenty of food. Plenty of places to study, live, etc. I live here. I can 100% recommend my place if you message me. Utilities included at a flat rate. Sleepy. Good area. If you're in NJ and not Cooper Village, you'll probably need/want a car. Not impossible w/o one (PATCO is 10 minutes walking from me as well and it's a 20 minute drive into CMSRU) but later on you'll spend money on Ubers in 3rd yr if you don't have a car and don't carpool (which you won't want to do).
In Cooper Village, it's literally across from the school. It's the city. It's a relatively safe area if you don't play stupid games. The PATCO is close by obviously for trips into the city. It's a food desert. Carpooling or weekly shopping runs w/ ppl is ideal. You'll essentially need to leave the city for actual groceries. The community in Cooper Village (COVID disclaimer) was, for us, great. The ppl hang out, carpool, etc. Having a car just makes it so convenient, both here and in NJ proper.