How long should a bandage stay on burned wound? ?

Until the wound has scabbed over and there's no longer a chance of infection

It should be changed every day or two, depending.  And it needs to stay there until it's started to heal.  Wounds need air to heal, but if it's an open wound, it needs covered

Are you doing this yourself? (If not ask the provider). There are special burn dressing, hydrocolloidal ones, follow the package directions precisely. Other things you can use are manuka honey and mepilex.

You need a special bandage for burns. Ask your pharmacist. An ordinary bandage is likely to promote infection. Small burns just need acriflavine cream and fresh air, no bandage.

Depends on a lot of factors.  In most cases, I would not put a bandage on a burn, it traps in the heat and can stick to the wound.