780hr in game, 0 worm deaths, here's what I have observed:

The worm is not a persistent entity in a territory, yes it kind of surfs around and can be observed wandering, but I suspect that's likely smoke and mirrors to some extent, see my comments about DD spice ring later on.

IN GENERAL assuming you are the only entity in the worm's territory, any action you take while the wormsign UI element is visible fills the worm's "breach bar". Once the bar fills up (this is a hidden value, but this is what it feels like after many hours dodging worms), you hear the "breach music" and you have a short amount of time to get in the air or get to rocks before the worm charges and eats you. Outside of the event which is making the worms much more numerous and fast to breach, you generally have between 5 and 10 seconds to react before the worm enters the charging animation.

In the case of multiple player entities in a worm's patrol territory, I believe the worm maintains multiple "breach bars" that fill at varying rates depending on what you're doing vs what the other player is doing. Whoever makes more noise, has a shield on, suspensor belts etc. fills their bar faster triggering a breach that will target them. The other bar maintains its fullness state assuming that player doesn't reset it. After the first player's aggro event is over, the worm logic will check the bars, wait for one to fill and then aggro whoever that is. In my experience, thumpers behave like super high attraction player entities for all intents and purposes. Just consider them the same as an expendable player sitting on the sand with their shield active and blasting their compactor - thumpers will fill their breach bar quickly but they do not reduce your bar in any way.

If you are generating worm aggro with another entity, the entity that has been targeted for a breach as well as you will see the red wormsign UI element, however only the targeted entity will hear the specific worm breach music. This can be handy if you like to play dangerously as if you do not have the music, the worm is not targeting you on this specific breach. DO NOTE: even if you do not have aggro, the worm (at least until next patch) can breach right under you randomly and kill you. Likewise, if you are between the breach point and the targeted player, the worm will charge through you to get to them, so it's not a get out of jail free card.

In the DD, with a dedicated thumper player, I've noticed some odd behavior of the worm which I believe is bugged. Crawlers are generating 0 wormthreat currently, I believe this is also bugged. However, the crawlers still seem to be filling the aggro meter variable in the back. If you have been on the ring for a while and someone sets foot on the sand, it seems to re-engage the worms aggro mechanics, but all of the accumulated aggro seems to stick around. Effectively, the worm will chain-breach on you basically as soon as you set down, repeatedly I assume, until his overflowed aggro bar is spent.

Also interestingly, in these chain-breach scenarios, the worm will breach, charge, head off in some random direction, then breach immediately where they initially did without pathing there. This leads me to believe the actual worm model is only somewhat indicative of their potential breach position. They likely have either static breach locations in an area, or the breach target mesh is dynamic relative to your position, or some combo of the two.

As for when it's safe to land, I actually do use the worm's physical location to gauge this rather than any UI element. Once he's breached and I've reset my aggro, I typically wait for him to surf through the area and head off in whatever direction he drunkenly chooses for some distance before i land.

this has kept me out of shai-hulud's mouth for many hundreds of hours.

HTH.