Once you have enough to support both of you for the rest of your lives, then by definition you don't need life insurance. So no later than the day you hit your FI number.
As far as the interim, it depends on what you are comfortable with. Paid off house doesn't, by itself, negate any possible need for life insurance, but paid off house + $500K in investments might for you or might not. If you have kids, that tends to push you towards the "need life insurance" side of the equation, but surely there is a level of assets where even with several children, everyone will be just fine financially if you die.
As an example, I'm currently a couple of years into a 10-year level term policy. I don't anticipate keeping any life insurance beyond the 10 years because even if we're not 100% FI, we'll be close enough to it.