This little radio really is excellent value for your money.
I was very surprised by the length of playtime I got simply from the winding alone.I believe that the running time depends on the volume you're using it at.
The product description says 20 minutes, however I accidently drifted off to sleep while the radio was on low volume and when I woke up some 5 hours later and realised my mistake it was still running!
Of course the product does have its downsides: the winding mechanism is rather stiff, the solar panel is only moderately effective in extending battery life if you live in colder climes and while the overall construction is solid it is also somewhat on the cheap side. You may also want to bear in mind that the radio is not waterproof, I keep mine in a plastic ziplock sandwich bag when taking it hiking ect. In saying that, for what you pay you get a good quality, reliable little radio with (literally) no strings attached. Well worth the price.
*Update*
Due to complicated set of circumstances involving a cat and a hoover, this radio ended up falling out of a first floor window onto a concrete path below. This slightly dislodged the dynamo, meaning that the radio made a horrible noise when being wound. But, after opening up the casing and pushing it back into place, the radio was as good as new. The case itself sustained only relatively minor scratching and nothing cracked or was broken. Needless to say it has proved itself to be a very tough piece of kit!