I took really good records on a long trip in 2019 so I often use these as a rough guideline - over 54 days of rides, I averaged 160 miles a day. This doesn't include the times I spent all day looking for a ride, just to have to find a place to sleep and try again the next day, so it might be too high. It also includes a lot of intentionally short or off-the-beaten-path legs so it might be too low. The longer your trip, the more likely you are to get super lucky sometimes, and the more likely you are to get stuck somewhere for days at a time.
Echoing what everyone else said, it's a crapshoot. It's my favorite way to travel, unless I'm trying to get somewhere by some date, in which case I spend the whole time stressed out. I think CA -> Philly I would estimate 2 weeks, but if I wanted to actually enjoy it and feel any confidence about getting there in time I would leave a month early.
Personally if I was doing this and trying to make good time, I would stick to interstates, and have a tentative route in mind but be ready to improvise and go off-route in favor of long rides as long as there's still a good way from there to Philly.