50 liters - But I wish I had more. Things like my air cut off tool really need higher free air delivery and a bigger tank, so the compressor isn't cutting in every 5 mins when you're using it. Still, it's enough for the impact gun and the air ratchet I've got. And it's great for keeping the tires on my Saab inflated.
If you are going to get one, get a 3hp oiled compressor (the oil-less ones don't last very long by all accounts) with at least 50ltrs - 75ltrs or more would be better if you have room (but then you start to trade capacity and portability off against one another). And check out the free air delivery figures - the higher the better, if you want to use high-speed cutting tools or do sand blasting.
I got mine off fleabay for £50, and a starter kit of air-tools didn't cost much more. Take a look - you may find a bargain.