just see what your baby wants - if he or she comes off after a reasonable length of time and still seems interested, offer the other side. If he/she is still on the same boob and happy then that's fine.
I always find that DS's swallowing starts to slow down after a while on one side - ie instead of swallowing after each 'suck' as he does at the beginning of a feed, he'll got to swallowing after perhaps 3-4 sucks. So when that happens and he starts to fidget I swap to the other side if he is still interested.
I was told that you can never fully empty a breast as you make milk as you are delivering it, but it does slow down, so that's when you swap.
I only ever really offer one boob per feed as this means DD is getting to the fat rich hind milk.
I was always told this little anecdote: When baby first starts to feed, your milk is like a drink and quenches their thirst so they will gulp it down. However, once this has gone your milk turns into the equivelent of single cream, so fat rich. The baby will tend to slow down their eating and pause between gulps to fully enjoy it.
Not sure how true this is but I have stuck with it and my lo is gaining well. I also feel the top of my breast once she has been munching for a while as this can feel a bit flatter compared to the other side which I guess could mean it's empty...
Perhaps someone a bit more knowledgeable will be on soon as would like to know whether I should be switching boobs more frequently than at each feed.