Each gravity point adds about 0.5 volumes when fermented. Typical longnecks are pretty safe until 3 volumes, and possibly can handle 3.5 if they're in good shape. Typically you're at about 1 volume after fermentation, so 3 points would lead to normal carbonation levels, 4 would be OK for Belgians and hefes, and 5+ would have me calling the bomb squad.