That depends on a lot of factors. Main ones include:
- Your low end target device
- an “iPhone 3G” is going to perform differently than an iPad3 or an iPhone 4s
- The resolution and types of textures you will be using.
- are you doing all textures or are you using normal maps? What resolution? 256x256 textures vs 1024x1024 makes a difference.
- The number and types of lights
- Are you using bone animations?
I suspect that with 1 model on screen, a background drawn you can probably spend 3000-5000 polygons and still easily get 30-50 FPS on an iPhone 4s. That is just an educated guess however. Your only recourse is to make a sample model and benchmark it yourself to see what is expensive to render and what is not.