Just Bento

I use egg frequently. It never got spoilt for me before. I would advice to cook it really well so there is no change for even a tiny bit to go uncooked. After a day carrying it around you shold not use it again, if you din't have the time to eat it. But keppt cool it can last a few days. Or so is my expirience.

Oh.... by the way it even worked in summer, and it can get really warm sometimes

Eggs (as tamagoyaki, iri tamago, usuyaki tamago, boiled eggs, etc etc...) are a standard bento item. As long as you observe basic safety precautions, they are fine.

- make sure you are using reasonably fresh eggs and good eggs to start with (Egg quality varies a lot by where you are buying it from and so on.)
- make sure your egg dish has been cooked through. Avoid runny-inside omelettes or soft-boiled eggs for the most part
- this may be the most important: make sure the egg dish, not to mention everything else in your bento box, has been cooled thoroughly before packing. Condensation from warm food trapped inside a box is one of the shortest ways to spoilage.

I don't like to make egg dishes too far ahead, because storing them in the refrigerator changes their texture. A day ahead should be ok though.

Eggs... in many forms... are a staple in my bentos: scrambled as a supplement to a meat soboro; hard boiled; or in a quiche or frittata. For some reason I think: once they're cooked, they're good. There's nothing in a quiche or frittata that, once cooked, would spoil.