I was able to cook after 5 months. Not all of the time, I still struggled with some lingering nausea and smell sensitivities but I was nowhere near as sick at that point. I found that most of my nausea was happening just in the morning and then at night when it was time for me to get to bed.
As for eating, I don't think I was eating a ton at 5 months but by the time I was about 7-8 months along I was eating CONSTANTLY and kept that up right up until labor (and even begged to eat during labor but at that time, it wasn't allowed though I believe they've recently changed those standards).
I didn't gain until the last two months and within those two months I gained 30 pounds (both pregnancies) so that's a pretty quick gain!