I have gone as high as 60. But the idea isn't necessarily to max out. Instead, you want to use strict form to tax the muscle.
If you do DB curls properly, you can get a burn with 30's. You might also try pairing curls with triceps exercises like pushdowns. One set of each and then rest. Repeat three times.
Something else I have tried occasionally is to "run the rack." That means curling with heaviest DB you can for 6-8 reps and then, without stopping, grabbing the next lightest weight and working on down until you can't lift anything else. This is not recommended on a regular basis but perhaps once every six weeks to stimulate muscle growth.