On Easy, Interest accumulates at a rate of one point per day (24 hours). It looks like the game starts with 20 Interest for some reason, so you should expect 480 days before the 500-Interest benchmark mentioned earlier. If I can do calendar math correctly, that should be July 14 2085, so I'm not sure what's up with Salvo's game.
I've seen them in 2.4, but I edited aircraftmanagement.ufo to spawn them faster and also increased interest using the debug_setinterest console command.
(Have I mentioned that the game currently has pacing issues?
)
~J