Actually, you can have some very large empires that generate huge amounts of culture and ripped thought the SP policies late in the game. I tend to play on marathon and have, late game, many times 20 to 30 cities generating 1500 to 1800 cultural points per turn - and a new SP policy every 25 turns (on marathon). It doesn't take long to fill out SP trees.
It really depends on WHAT you decide to concentrate on building. If you decide to build cultural building AND target SP/Wonders to help accelerate or reduce cultural cost, you can generate large amounts of culture with large empires. And you can generate the culture faster than the next cultural level goes up. But, you will give up the ability to generate massive production, or massive cash, or massive science. Its all about trade offs.
But, your producing every cultural building you can in every city. And you have to decide if that's the wisest course given your particular situation.