1: "Average" render time is extremely hard to judge because it depends not only on the details of the scene setup (how many cloud layers, how much displacement, etc.), but also the specs of your machine of course. I suggest checking out our benchmark results and running the benchmark yourself on the TG3 Free version:
http://planetside.co.uk/products/tg3-benchmarkUlco's examples of render times are reasonable. With a rather simple scene (simple texturing, but lots of tree populations perhaps), you could certainly have 20-30 minute renders at 1080p, depending on settings (e.g. some trees are harder to AA than others).
2. As Ulco mentioned, OBJ is currently the best method to use for importing objects. OBJs must have an accompanying .MTL file to assign texture references and UVs.
3: The render engine is CPU-only. CUDA and other GPU acceleration is not going to be available in the near future, although we are certainly interested in it.
4. You cannot export a complete scene at this time, nor are there any plugins for other renderers. You can export terrain-only however (no textures). Terragen's procedural functions, texturing, and atmospherics (volumetric clouds, etc.) are all finely tuned and developed specifically for rendering within Terragen's proprietary render engine. You will always get the very best results rendering these elements in TG. If you want to render an object that TG doesn't import very well, or some element like hair that TG doesn't currently support, your best bet would be to export a spherical environment map from TG and use that to light your object and render it in another app, then composite that with the TG rendered backplate.
I definitely echo Ulco's recommendation to try the free version as it is a good representation of how TG works and what you can achieve:
http://planetside.co.uk/products/download-terragen-3- Oshyan