You wrote K5 above but posted in the K3 forum. I assume you want to know about the K5 (the K3 is different, but similar). My short answer is ISO 3200 although it depends on the scene.
I'll readily go to ISO 800 in low light. If the scene is not moving I use a tripod + lower ISO + slow shutter. If the scene has movement that requires a faster shutter speed, or I don't have a tripod available, I get good results for on-screen viewing up to ISO 3200.
As you increase ISO, the dynamic range and colors suffer. Colors look off at ISO 6400 so I convert to B&W as needed.
Shoot in raw DNG not JPG for maximum post-processing flexibility. Noise reduction software works much better with a DNG.
Here's a sample of ISO 3200.