No. If everyone were allowed to do so, the "new" cars would get worn out by people testing them.
An hour is the standard test drive, depending upon the dealership.
You should drive all of the vehicles you're considering on the same day so that you can compare them more easily. Drive them as long as possible—at least 30 minutes—and over different types of road surfaces and in various driving conditions. Plan your own driving route in advance. A salesperson or private seller may suggest routes that hide or minimize problems.
Some dealers will let you take the car out by yourself, but don't be surprised if someone comes along. This is where your companion can come in handy. Let him or her deal with a jabbering salesperson as you concentrate on the vehicle.
But this can be a potentially dangerous situation. A new car is likely to overly impress you, because it's better than the one you're now driving. It's important to compare it with other new vehicles that interest you rather than with your current car. The more vehicles you test-drive, the better perspective you'll develop.
Listen and look for the things you like, and be on the lookout for things you might not be happy with. Many cars have personalities, and it's important to find one that matches yours. Little things that might seem insignificant now could become major irritants down the road.