The frequent delivery principle in agile methods tells us that we should deliver products to our customers frequently. Quite often a fixed release schedule of X months or Y weeks is advised.
At my company we build and maintain an intranet application that is released whenever we feel like it, sometimes as often as twice per week. It depends on how valuable the new features (and bugfixes) are to our users. Why wait for a fixed monthly release date when a new version can be deployed to users in just 30 minutes?
It now appears that our practice leans towards the "new" idea of continous production, which states that you should deploy new versions as soon as possible, and as often as you want.