This is the doomsday scenario: bitcoin's market cap goes into the trillions of dollars and the cost/benefit ratio of cracking the encryption algorithm tips so that it becomes attractive to throw massive computing resources at a hitherto-impossible scale at the problem.
The same encryption secures the websites of the world, so there is currently a pretty good reason for "cracking" pools to form. But there is insufficient centralization of value, plus human intervention, to prevent the full scenario to unfold - the web would be shut down before all accounts everywhere were drained. These two factors are largely absent with bitcoin.
A round number, within a few trillion dollars, will be sufficiently precise. If there are too many variables, or you have no idea, then please feel free to just make up a number!