How does an index's document count affect performance?

Are there any metrics, posts, or otherwise good information regarding how document size as well as the total document count on an index affects search performance? I understand other mitigating factors such as mapping, analyzer, etc., but how do the sheer numbers line up?

For example, I have a basic mapping with 5 general fields, no nesting, or anything special. How can I estimate search performance would be affected from say, 100k documents to 100 million or 1 Billion documents?

It can depend on a number of factors, so the best way to know is to benchmark using realistic data and queries on your hardware.