I did some math (bear with me)
Okay don't quote me on this but I wanted to see how long the trip to Erid actually took (I know I could look it up but I'm bored it's summer and I want to do math)
So to begin, Sol is 11.9 light years away from Tau Ceti, this is an established fact in the book and movie. For my dumb dumb math I'm going to say the trip to Tau Ceti took around 4 years. With that in mind, 11.9/4=2.975 light years/year.
Now, if my memories serves me correctly, it's never stated how far the distance is between Tau Ceti and 40 Eridani, so I just looked it up and it's about 10.02 light years. Now, looking at the ly/y, the equation is simple. 10.02/2.975= ~3.37yrs.
So we know that around 3 months of actual food was packed for each of the crew members. To make myself feel better, I'm going to say that Grace simply was at Tau Ceti for the expected three months so his food supply is gone. So he has around 6 months of solid food, during which he was likely fine health-wise. I'd say he ate two meals a day to stretch it out, so that's 9 months out of about 40.44.
(this next part is fully speculation and assumptions as I'm too tired to look up what's in nutritional coma slurry)
Once that's out though, he's left with coma slurry (ew+meant to sustain bodies that are doing nothing but sleeping) and taumoeba (no nutritional value). This is when he started getting sick.
For more stability, I'm going to say both Ilyukhina and Yao died around the halfway mark, which gives Grace around four years of extra coma slurry, but again, that stuff was meant for people that were never moving and basically only burning calories by digestion. Meanwhile, Grace is moving around, talking, etc, summed up he's burning a lot more calories. What could've been stretched into years in a coma he likely burned through in a couple months. I'd estimate that it would be around 20 months, so that's 29/40.44.
Additionally, the coma slurry is gross. As described in the book, it has a chalky texture, and it tasted like aspirin. If Grace's stomach is as weak as he says (I get sick on an elevator) then he likely couldn't hold down the slurry a lot of the time, which would waste it. (If he can't handle eating coma slurry mixed with taumoeba, imagine that mixed with bile now. Ew.)
Now, the Taumoeba. After he runs out of the coma slurry, taumoeba is the only thing he can eat to stop his body from simply shutting down and eating itself. This is when he gets Scurvy. The constant Taumoeba breeding likely means it can last him until Erid, but it would be miserable.
A bit of a TW for this next part, suicidal ideation
I can't imagine how that would be, almost a year of just watching as you deteriorate. Your body becoming weaker by the day, freezing, bleeding, constantly in pain. I imagine that during that Grace used every ounce of his strength either crying or looking for the Heroin/Gun. Rocky likely would be just watching Grace sleep and trying to keep him happy whenever he could.
In conclusion, I'm tired and Grace is a perfect angel that deserves no pain
There's that to cheer you up, little astrophage having a snack, he's so jolly