An average of 60-70 characters is ideal for desktop viewing.
40-50 is more realistic on mobile.
Line length (measure in typographer speak) is an important factor for comfortable readability, especially if you expect prolonged reading (pretty rare on screen). James Craig said it well in Designing with Type:
Reading a long line of type causes fatigue: the reader must move his head at the end of each line and search for the beginning of the next line.… Too short a line breaks up words or phrases that are generally read as a unit.
Measure isn't everything
Example taken from Pocket mobile. Notice the relatively short measure.
Measure has to be considered in light of other factors that are of equal or greater importance.
The key determining factors when designing for readability are:
- Font size
- Letter spacing*
- Word spacing*
- Foreground / background contrast
- Leading (space between lines)
* Side note on letter and word spacing:
This shouldn't be big concern, as long as you use a good typeface and set it at the right size. These factors are usually accounted for in the font itself and rarely require adjustment.
The not-so-special case of mobile
Considering all factors, you'll find that mobile readability is best with a shorter measure. This is nothing new. Historically, measures in the 30-50 range have been common for multi-column layouts like magazines and newspapers (remember those things?).
In those contexts, multiple columns require narrowing the line — but you can't just keep shrinking the type. It's better to have a readable type size (and leading) and live with a shorter measure. This format would cause reader fatigue in extended "immersive" reading, but that's not an issue for the casual type of reading happening here.
Mobile fits the same scenario.
Smashing Mag has a great primer specifically addressing line length factors and responsive design. Read that whole thing before you make any further decisions.
There is also a long in progress work translating Robert Bringhurst's classic Elements of Typographic Style to a web-centric resource: Richard Rutter's The Elements of Typographic Style Applied to the Web. 👈 That link will take you to the section on choosing the right measure. One great observation from that page you may want to consider:
… the beauty and advantage of the Web as a medium is that readers are able to adjust their reading environment to suit their own needs. This is a concept that should be acknowledged & embraced, and built into website designs from the ground up.
I addressed a similar question here some time ago.