what is the lifespan on a iphone 5s?

I guess it would depend on how well you take care of it. If it helps any, my original iPhone still works, as does an iPhone 3G that I had.

Until it stops working. There's no way to predict that. There are people still using iPhone 3G phones. To some extent, it will depend on how well you take care of it. Eventually, the battery will stop holding a charge and you'll have to get it replaced. Eventually, you will be unable to update the iOS on the device. Eventually, there will be fewer and fewer apps available for the phone.