This is the guide that I use. I belive it to be as precise as anyone can get without sitting at the entrance and counting the ship mass and time. I think its designed that way so that there is always a bit of mystery about WH's. That's part of what helps keep it interesting.

The important bit pertaining to your question is:

For time collapse:

• "Life cycle has not begun" - This means you've found a wormhole just as it is forming, and it will spawn in a minute or two.
• "Probably won't last another day" - This is the wormhole's main life, and has over 25% of it's total time remaining.
• "Reaching the end of its natural lifetime" - This signifies the wormhole will be naturally collapsing soon, having less than 25% of it's total time remaining. Collapse is imminent.

For mass collapse:

• "Has not yet had its stability significantly disrupted" - This indicates the wormhole is still quite stable, and has over 45% of its total allowable mass left.
• "Has had its stability reduced, but not to a critical degree yet" - This means the wormhole has seen a decent amount of travel, and has less than 45% of its total allowable mass left.
• "This wormhole has had its stability critically disrupted by the mass of numerous ships passing through and is on the verge of collapse." - This means exactly what it says. The wormhole has had most of its total allowable mass used up, and has less than 5% left. Collapse is imminent.

(edit: formatting)