If your goal is to be away from cities, traffic and crowds then its a fairly simple formula.
If something is convenient, you're too close.
In our situation we are about 20 miles from town (not a city) but a small town. What most people would consider a "city" is about 60 miles away. Personally from my perspective its not really the mileage that is significant. Its the access that is more significant. If there are major interstates connecting your world with other areas the access for "outsiders" is fairly easy. Just keep in mind even if the world doesn't continue to melt down you will eventually need access to medical attention and items that you can't grow or make yourself so your proximity to these things is also important.
We live in a pretty rural area and have quite a store of our own supplies and a reasonable ability to generate our own food. However, some things are finite such as fuel, tools, parts and medical supplies. Its just a fact of life that there are things I can't reasonably make for myself such as tools, screws, bolts, nails, lubricants for equipment, parts (tires, belts, hoses), etc. I can graze some of my animals but not over winter so feed would be needed at some stage. We have a large water well (requires three phase electrical) but keeping that going without utilities would be difficult. Being 100% self sufficient would be nearly impossible, certainly I could barter food and other items for things I need but without a modern supply chain life would significantly change. I believe the further a person is from even minor population areas the tougher these tasks become even if the world just changes and doesn't melt down.
I'd always encourage people to pursue what is in their dream/adventure. From my experience there is more to the equation than just finding a remote location where a person can hide from society and build their homestead.
Stay safe & healthy