Knowing the animation and the attack cycle is the key to successful blocking. The wurm has several animations:
Many of these animations are present at Great Jungle Wurm at Caledon Forest.
Spawn 3 husks, one per direction
Cooldown: 100s for Crimson/Amber, 90s for Cobalt
The animation you should be on the watch are husk and egg spits. They use the same animation, but has varying cooldown. With this difference in cooldown, a cycle can be derived:
Note that once both husks and eggs are off cooldown, it can use both skill in any order.
The wurm does not operate on a cycle, rather the cycle is a simplification of the cooldown. If animation outside of the cycle (eg. burn phase, eating the zerg) happen the cycle could be inaccurate.
This video demonstrate a blocking attempt at Crimson. A husk and eggs cooldown timer is provided, but should be disabled to simulate in game attempt.