I would venture to state that 20% is a basic compliment for shields. If less than 20% of your ship blocks are shields, you are light on shields. If substantially more than that of your ship is shield blocks, then you are starting to get shield heavy.
Of course as others have pointed out, it is not as simple as that. A ship with heavy armor, with multiple spaced layers in the front, running maximum pierce and punch protection, could easily get away with 20% in shields and still be considered an extremely survivable ship. On the other hand, if your ship has little to no such advanced armor mass, you would probably want to go for as much as 40% of your ship blocks in shields, if you are building a combat ship.
What ratio of shield capacitors to shield regenerators you should go for within those percentages depends largely on whether you expect the ship you are building to have to deal with PvP or not. If you are going up against players, you probably want to emphasize shield capacitors as the damage output of their attack will likely be far more than anything shield regenerators could cope with, going for an 80% capacitor to 20% regenerator ratio. If you expect instead to be largely dealing with pirates and NPC ships, where you could find yourself up against larger numbers and successive waves of less potent ships, then you would probably want to go for an even mix of capacitors to regenerators.