You get a 1% chance for a while, even a 1% chance is still a chance.
Not sure how long the 1% chance goes on for, but level 30 grifts give a 1% chance on level 45 gems, so I think it's safe to assume level 50 grifts should give at least 1% for a level 65 gem
As I'm typing my bot is running in circles trying to discover the map. I don't understand how anyone can complete >35 in time without intervention.
It also depends on mob density.
But looking at the bot playing at Grift 41 is so funny because it dies like once every secondI really admire those who can do Grift lvl 50+++ manually....