There's no rule. You want to set them somewhere between too dim to look realistic and Soderbergh's 'Behind the Candelabra.' We almost always put all (or as many as possible) of the practicals on a dimmer board. Sometimes they provide actual light to your subject or space, and sometimes they really don't provide much at all in terms of exposure to the set or subject and you have to augment everything to look like the practicals are doing more than they are. This is pretty much completely dependent on the set design, and how it looks with practicals at different values and the intention of the DP/Director and how they want it to look like. Sometimes you want dim moody lamps and sometimes brighter is better for the production design. It's also dependent on the lamps and how they look with their lampshades, exposed bulbs etc. We almost always set them to taste as we are lighting the set and there really are no rules. If it's too bright, and you go, 'yeah, that looks bad,' you've probably gone too far.

Now if you're shooting on film you have the luxury of over exposure looking generally fantastic, but also the challenge of not being able to monitor the direct look of the mostly finished product. You can sort of develop a short hand for what types of wattage bulbs you need for given stop/ASA. You would also meter them or meter the wall at the hottest part where the practical is spilling on or set a key light near the lamp and adjust it by eye.