The fibroid should be affected long before the fat runs out. After all, when you run out of fat, you enter into REAL starvation. Yet people report all sorts of growths and such being autolyzed long before starvation sets in.
I recall Shelton saying one patient had a patient with a grapefruit sized fibroid disappearing after something like 21 days (which he thought was was faster than normal) yet another patient had some other lump about half the size but firm shrink in 28 days (which he thought was too slower than normal). So if his idea of how fast these things should shrink is correct then 40 days should be more than enough.
Just don't fast 40 days for the sake of 40 days. I strongly agree with Shelton's idea that the fast end when hunger returns. So if it returns on day 38, end it.