Where are you going to and where from? If you live in the City and commute to Silicon Valley forget 20 minutes. If you need to cross the bridge, either of them, it's longer as well. If you live in outer sunset and need to go downtown how are you electing to travel? Muni or car? Depending on where downtown it can take well over 30 minutes. If you live closer in, it's considerably shorter.
I guess that unless your question is more specific, it's impossible to give a good answer to it.