Setting the line at a release every three months can seem a little arbitrary (and in some ways it is). Depending on where you are coming from, you probably will react in one of two ways:
If you are starting a large complex project, getting the first release out in three months will strike you as an impossible and naive goal. The important thing to realize is that even if it isn't feasible to release after three months you should be working towards that goal. As long as you are not able to achieve a three month release cycle you have to accept that this will lead to a higher level of risk in the project.
If you have a system that is already in production and have implemented Continuous deployment you will be releasing much more often than every three months. This is good, but how long was the first release? It is also important to remember that a release cycle of three months means that every feature that is developed is in production three months after development started on that feature. In other words, you can be releasing every month and still have a release cycle that is longer than three months.