If you don't wean the calf before she has the next calf, she won't ever dry up, just continue to milk. Some breeds (Brahman) will wean their own calves when they're close to having the next, thus avoiding having a new calf, and last years nursing together!
You can wean the calf at whatever time works for you.. depending on what you're going to do with the calf. If it's a terminal calf, and it's going to the auction barn, wean the calf based on the cow's condition and prices at the barn. If you're keeping it for a replacement, a lot of producers like the cow to nurse the calf for a longer period of time, because generally the calf does better the longer he's on the cow.
We wean terminal calves at 5 or 6 months. If the cow was late getting bred back, we have left the calf nurse for up to 7 months.