I had a cheap one that would only work a few hrs in the summer.
I ended up getting a larger unit (8 leds, 240 lux) that would last all night with the exception of a few weeks in the winter. Of course getting the most direct sunlight is a plus and the angle of the solar panel helps. I change the angle in the Spring and late Fall to help keep the most effective angle I can.
You will pay for what you get but to me something like this is not about the money.