If you post a picture we may be able to give you an approximate voltage range.
The higher the voltage you feed a brushed motor, the faster it can spin. Off the top of my head there are three things that limit the maximum voltage you can run a brushed DC motor at:
- Maximum heat dissipation. Motors heat up, especially if they are doing lots of work. eg if your motor is driving something uphill then it will get much hotter than if it is spinning with nothing attached.
- Maximum insulation and creepage voltage. Generally this is way above the voltage you would normally run the motor at (well above 20 or probably even 50V)
- Maximum mechanical speed. At some point the motor will self destruct. At different speeds and conditions it will wear at different rates.
Most often the first problem of thermal limitations is what you hit first. Power that can be dissipated (heat) increases dramatically as you increase the voltage. Most small, cheap, brushed DC motors will happily run on 5V without problems, provided you are not stalling them.