While there’s no exact criteria for becoming a committer, there is a fairly obvious path to becoming a committer.
For starters, one should be familiar with the Apache Way , especially the part about meritocracy.
Second, participate in the mailing lists, help answer questions when you can and do so in a respectful manner. This is often more important than writing amazing code.
Third, write code, add patches, stick with them and be patient. Add unit tests and documentation. In general, tackling 3 or 4 decent patches is where the bar is at, but it depends on the state of the project. In the earlier stages of the project, the bar is a bit lower, so it pays to join early!
Finally, it is then up to someone to nominate them to the PMC. Typically, one of the existing committers does this by sending an email to the private PMC mailing list (private@m.a.o, where m.a.o is mahout.apache.org) and then the PMC votes on it. Nominations often occur internal to the PMC as well.