I think a plant in a one gal container should top out around 2 feet, give or take a few inches. If I have clones in a 1 gal pot, I'll flip them at no taller than 12 inches, and that'd really be pushing it.
If growing from seed, they should be flipped probly no taller than 6 or 8", as plants from seed tend to grow what I'd call "robustly."
Obviously, it kinda depends on the plant; it would help to know what you're growing. Something that gets stretchy and tall in flowering should be flipped small; something short and squat should be vegged to almost full size before flipping. Research your strain, find out what kinda plant it is, and proceed accordingly.