Can it be done at any time before the next salah begins (providing it isn't during a forbidden time), or should it be done as soon as possible? Is it sunnah to pray as soon as you can, or fard/wajib?

Can it be done at any time before the next salah begins (providing it isn't during a forbidden time), or should it be done as soon as possible?
Is it sunnah to pray as soon as you can, or fard/wajib?