how long should a nightlight last?

Those little incandescent bulbs never last long.  Get an LED nightlight and forget about it.  

The average nightlight will perform for seven to 10 years. Durability is usually tied to price; nightlights with shorter lifespans are often the cheapest models available, but replacing them over time can be more expensive than investing in a more durable model at a higher price-point.

what a question may as well how long is a piece of string , or as long as the warranty is .

Frequently MEANS EXACTLY WHAT?   2 years is a decent run.

I bought one from dollar tree about 2 years ago, that has a sensor to go off when a certain amount of light hits it. Surprised it still works, honestly

I do not know what this night light is, but it has been plugged in and burning for almost 20 years. It is flat and looks to be a flat piece of plastic that glows blue and serves as a focal point more than a night light.

Neon or LED lights will last a very long time, years even decades.

Incandescent lamps may last 6 months to a year.  

depends on the type of bulb it uses.  if you buy an LED nightlight, it should last for many years -- get the one with the senosr and leave it on all the time

Junk from China get's cheaper all the time. Mine lasts a year or more and comes on by itself in the dark.

Leave it on all the time, it will last longer. But it will uise more power. Consider switcxhing to a low energy bulb e.g. LED