You could batter fish ahead of time before cooking and serving it.
But most people just batter the fish 30 minutes or so before they cook it and serve it.
For the best fish you should batter the fish within 30 minutes of cooking it and serving it otherwise the fish may not be as good tasting as it should be.
If you do batter your fish ahead of time make sure you refrigerate the fish promptly and then cook it as soon as possible the next day or when you plan to cook the battered fish.
I've always just battered the fish and cooked it because it doesn't take that much more time the next day to prepare the fish or even the same day.
I like the battered fish to be the freshest I can get it so I always batter the fish and cook it the same day.