From: How to read an English story book to the kid when we don't talk in English at home?
If you start immediately, it won't matter what language you're speaking; either way it will be babble :) Language comprehension takes time, regardless of whether the exposure is monolingual or bilingual. But early exposure, even in the womb, has been shown to increase a child's level of responsiveness to specific languages, which (presumably) facilitates learning the language later on.
It seems to me that if I start this as a ritual, I'll feel like talking to walls since I will know that the kid doesn't understand what the hell I am speaking and won't be responding either.
Hence the question: How to motivate yourself to read stories to a kid whom you know cannot understand and respond accordingly?