Sometimes a version is just a "bugfix" version because a number of small and big bugs were found, although those versions do tend to include one or two new features.
Major updates (i.e. the transition from 6.03 > 6.1) often contain big new features, a change in GUI, bugfixes of course etc etc. It took 1 year to go from 6.03 to 6.1.
As you can imagine, as Khaled does not release information about what will be included in the next version it is hard to determine whether the next version will be a major one or a minor one. He also does not release information as to when the next release of mIRC is scheduled for or what it is called (for example, a lot of people expected 6.13 but the next version was 6.14). Therefore, there are never really "regular" updates as that would suggest some sort of general guide as to how long there is between releases. The answer is, Khaled releases a version when he feels it's necessary and ready for release. If he does have his own private target date for the next release then he doesn't make it public knowledge.