It's never really a good bet....
However, if you take into account the number of dreamers hoping to hit the jackpot, then I'd say whenever it gets up and over 330K.
I personally am not going for the jackpot, (although it would be nice).
I am going for the players at the jackpot tables....
I also tend to chat about how great it would be to hit the jackpot in the hopes it gets these players thinking about it and playing there 3-4suited to the river in the hopes of making that runner runner gut-shot straight flush draw.