Each expansion lasts 2 years, consisting of 5 major patches that are 4 months apart.

5.0 - item level cap raised from 400 to 470, new crafted gear/normal raid gear (450), new tomestone/gear (460), new savage raids (470 gear), level cap raised (obviously)
5.1 - catch-up patch so non-raiders can reach item level cap. new 24-person alliance raid (casual content that drops 460 gear and an upgrade to tomestone gear to 470), you can farm hunts to get this upgrade too.
5.2 - item level cap raised from 470 to 500, new crafted gear/normal raid gear (480), new tomestone/gear (490), new savage raids (500 gear)
5.3 - catch-up patch so non-raiders can reach item level cap. new 24-person alliance raid (casual content that drops 490 gear and an upgrade to tomestone gear to 500), you can farm hunts to get this upgrade too.
5.4 - item level cap raised from 500 to 530, new crafted gear/normal raid gear (510), new tomestone/gear (520), new savage raids (530 gear)
5.5 - catch-up patch so non-raiders can reach item level cap. new 24-person alliance raid (casual content that drops 520 gear and an upgrade to tomestone gear to 530), you can farm hunts to get this upgrade too.

4 months later is 6.0, same system for the 6.0 patches.

The patch cycle has only been delayed once because of COVID-19, which caused 5.3 to be 2 months late. There are lots of smaller patches and they may include other things like PvP, blue mage, relic weapons, story content and dungeons, but I'm specifically explaining the item level and tomestone progression.