they're generally every 30mn, with a journey time of just under an hour. how reliable? well you need to bear in mind this is "Southern", aka Govia-Thameslink, who run this service, who are somewhat inept as well as being in dispute with both main rail unions. if reliability is what you need above all, it might pay you to travel from three bridges (from where thameslink trains also run).
Only 77.3% of Southern Mainline trains arrived at their destination within 5 minutes of their scheduled time yesterday (with only 45.5% arriving within 1 minute of the scheduled time).