How do I know when my baby's finished his breastfeed? Should I always give two sides?

You won't always know! Sometimes you'll get it wrong. You'll think your baby is full, satisfied and sleepy - but as soon as you grab the chance to move him away from your breast, he'll wake up and ask for more.
Or, he may be asleep in his cot after a feed which has seemed to fill him up, and cry and fuss after just a few minutes, only to act ravenous when you put him to the breast again.

It doesn't really matter. Your baby is the best guide to 'how long to feed' - after all, babies can't tell the time, and the number of minutes on the breast doesn't correlate with the amount of breastmilk taken anyway. And even if it did, we have no way of knowing how much any individual baby needs at any individual time.
Let your baby properly drain the first breast first. That means, let him feed for as long as he seems to want to on the first side. If he stops sucking and comes off and looks sleepy and 'zonked', use this time for a natural break in the feed. Sit him up, or place him looking over your shoulder, and rub his back if you like. He may or may not burp (it doesn't matter either way). If he still seems sleepy, you could perhaps change his nappy to wake him up a bit. Then, offer the second side, which he may or may not take. It's a good idea to start the next feed with this second, or unused, side.

Letting your baby set the pace this way means you let him get the calorie-rich hindmilk that comes towards the end of a feed. The fat content of this milk may be as much as five times higher than the foremilk which comes at the beginning of a feed. Taking him off before he's ready could mean he doesn't get a satisfying feed.