I have lived in several states and always checked into the plates for my Firebird in each one not knowing when I would get it back on the road. Usually, a car is eligible for "antique" plates if it is 25+ years old and is insured with classic car insurance (i.e. Haggerty). The mileage restrictions are usually either from the state itself or the insurance company but they commonly center around not using the car daily or for "commuter" use. In other words, used solely for pleasure and such things as attending car shows. Some states, like here in AZ, also exempt you from smog testing (excellent).

I consider the "antique" plates here in Arizona to be just butt ugly and the ones in Oklahoma were plain and boring vanilla white, so I have always researched further into "Year of Manufacture" plates for each state. I have picked up quite a collection from Ebay as I kept moving around.

But as mentioned above, we can generalized all we want to - but you will have to check the specifics for your state.