I think I would gauge it on your needs. It's easy to start small with a bottle and cage. If you begin to find that:
1) It's not enough water for most of your rides or
2) You go through slop that gunks up a bottle with dirt and cow crap on a regular basis or
3) You find yourself wanting to carry a multitool, tube or patches, pump, maybe car key and wallet and it's starting to get out of hand with an under-saddle pack and two water bottles and your light system and gawd knows what else hung all over the bike and it's time to consolidate.