It really varies by exactly what it is, and what temperature it's meant to be served at.
In general, when the box says 'keep frozen', the cooking instructions assume that the item is frozen when it goes into the oven unless it specifically has multiple sets of times & temperatures.
If it's an item that's meant to be served at either fridge temperatures ...
If you bought them frozen, you can keep them frozen, possibly for months, until you want to eat them.
If you bought them already defrosted, you should eat them within the next few days.
You could refreeze them, but refreezing often affects the texture of many foods (the package will usually say "do not refreeze").