I know this sounds stupid, but when do you expr...

I know this sounds stupid, but when do you express to store EBM? Do you express after the BF is completed or wait a while? I would like to express some milk so that its there for an emergency or if I wanted to go out and leave bub with a sitter. I would have thought that if you express after bub has been fed, there wouldnt be much milk to express. If I wait for an hour after the feed, is there a chance that I am expressing the milk that bub needs for the next feed and he wont get enough? Last stupid question... how much EBM do you need to feed an 7mth old? Is there a site that anyone is aware of that could help? Thanks