Hi and welcome to Blenderfarm. Hope you like it. We really look forward to your feedback, since we want to make Blenderfarm into a very useful tool for us all. There are still some issues we are working on:
1) The calculation of estimated render time is based on Cinebench benchmarks for basically all known and relevant CPUs. We then compare this with the time it takes for Blenderfarm to render the same image.
Now, this works well in most cases (though we are still calibrating the system, so please let us know if you find our calculations off for your own CPU). Yet we have found that for certain very complex scenes, the ratio between GPU and CPU rendering time is different from other scenes. So we realised that it is pretty hard to make an accurate calculation like that, between CPU and GPU. But we are trying our best. In any case, you know what you get and it is just an issue between Blender, the GPU and your scene. If the job takes longer than expected and you run out of time, you will be notified by mail and can schedule a new time to continue it. So bear with us and don't be upset if that happens. You don't pay for a job within a promised time. You pay for using 20 GPUs (currently GTX 970) to get your job done, and they will do it the best they can.
2) Blenderfarm is still optimised for animations. We plan to implement tiled/split renders across several machines, to also render very heavy still images with full capacity. For now on, that should make no difference to you as long as the render time on your CPU is less than 8-10 hours for an image. You will then in any case pay only the minimum charge of one hour. If an ordinary CPU renders your image in up to 16-20 hours or so, you would typically pay for two hours at Blenderfarm. Something like that. If there are many Blender artists who want to use Blenderfarm for stills, we will make it more efficient in that respect. Let us know if you are one of them.
3) This forum is still a bit buggy. We'll work on that.