Several 20 minute sessions can often be more beneficial than a single longer one. We generally gain the most benefit in about the first half hour or so, and stepping away gives our brains time to absorb what we've just done. Each time we return and recall it, it becomes that much more permanent.
Of course, there are some things that we just need to practice doing over and over again, and those may benefit from longer sessions. For the most part though, 20 minutes of focused practice at a time is pretty ideal.