That all rather depends upon what the brew is, what other nutrients you may have added, what yeast you're using, what the temperature's like, or if you've added anything which might inhibit the yeast.
If it's under an airlock I would assume it's a wine. Even when pitching the yeast dry, I would expect a Wurzel's Orange to be bubbling within the hour. Other wines may take a few hours before they start to show any signs of activity. I've got 5 gallons of Damson at the moment which flatly refuses to ferment. I must have overdone the sulphite when washing the fruit. If it isn't going by tonight I'm going to have to make up a new starter bottle and try to get it going gradually.
Edit: Sorry, just notice this thread is filed under Beer. I would expect to be seeing a head of foam, certainly within 12 hours. I don't use an airlock on my bucket, the lid isn't gas-tight.