Path Tracing is a rendering algorithm similar to ray tracing in that rays are cast from a virtual camera and traced through a virtual scene. The rays are distributed randomly from within each source pixel and at each intersection with an object in the scene a new reflection ray is picked at random. Each ray eventually exits the scene or is killed of. When a ray has finished boucing about in the scene a sample value is calculated from the materials the ray bounced against. The sample value is added back to the average for the source pixel.
The random components in path tracing cause the rendered image to appear noisy. The noise decreases over time as more and more samples are calculated.
Render Quality & Samples Per Pixel
The defining factor for render quality is the number of samples per pixel (SPP).
A simple way to understand SSP is to imagine a camera in a dark room taking a long exposure shot. In the room there is laser that shines white light. The laser quickly switches on and off, while also rotating wildly so that each time it is switched on it points in a random direction. Each laser light burst is one sample, if enough of them are seen by the camera during the exposure you will get a nicely lit photo, assuming that all samples were uniformly distributed.
The higher SPP you have in a rendered image, the lower the noise, or graininess, will be noticeable. However after a certain point, there will be diminishing returns. The difference in image quality between, for example, 20,000 SSP and 21,000 SSP will not be as noticeable as between 1,000 SSP and 2,000 SSP.
Sunlight does not require high SSP to give a nice image. Outdoor scenes can be rendered with relatively low SPP if sunlight is enabled. Emitters (torches, lava, glowstone, pumpkins, etc.) require a lot of SSP to reduce the noise, so indoor scenes and similar scenes in low-light environments take a much higher SPP number to look good.
An indoor scene rendered with increasing SPP. The white numbers show SPP:
An outdoor scene with sunlight and emitters:
An outdoor scene with sunlight and disabled emitters:
There is no definite answer to how long it will take to render a scene. The general guideline is that the longer you render an image, the better it will become. Take into account the diminishing returns explained above.
The time required to render a nice looking image depends on how well-lit the scene is, how many samples per second the renderer can produce, how powerful your CPU is, and how large the canvas is (in pixels).
Scaling the canvas has an effect on render time proportional to the pixel area of the canvas. An image of 800 by 800 pixels will take four times as long time to achieve the same quality as an image of 400 by 400 pixels since the total number of pixels has quadrupled. So, if your renders are taking too long, you can try to reduce the canvas size for quicker results.
There is a handy online calculator available that will show you an estimate of how long time it takes to reach a required SPP value here: SPP Calculator
Noise from torches
Small but bright light sources, such as torches, add a lot of noise to a scene. It takes especially long time to render a scene that is lit mainly by a few torches. This is an unfortunate and unavoidable disadvantage of the Path Tracing rendering method.
The reason for this effect is based on the low probability for each sampled light path to include the torches, versus the high luminance of the object. The final render takes the average of all sampled values, but the average can be "too high" for a long time because of the high luminance. The average will decrease over time, but for a while there may be one pixel that has been lit by a particular light source in a neighborhood of several pixels that will stand out sharply against the others that have not yet been lit by the same source, hence the bright dots seen above at low sample counts.