If I'm adding a toilet, is there a limit to how far it can drop before connecting to the vent pipe?

Yes, indirectly. The UPC doesn't list a specific 'maximum vertical drop' for a toilet before its vent, but it limits the horizontal trap arm length (typically 42 inches for a 3-inch toilet). If the drop is too long before connecting to a properly vented line, it can behave like an unvented, oversized trap arm, leading to siphoning or pressure issues. For wet-venting, proper sizing and connection points are critical to avoid problems. Always get a permit and use a licensed plumber for new toilet installations to ensure code compliance and proper venting.