I use a Sawyer as a gravity and inline filter; one Sawyer mini was fine for 4 of us for a 25 day JMT hike and is still in occasional use. It's plumbed into my drinking bladder hose; "dirty" water goes into the bladder and I drink through the filter as an inline (a bit more effort than normal, but not too awkward). When we stopped for breaks, lunch, camp I'd refill by bladder with dirty water, put it on top of a rock, or hang it from a tree, connect it onto a 2.5L platypus and fill the 2.5L platy with clean water that the family could then refill their bottles from.
I have no precise idea how fast it filtered, a) because when it's happening in the background while I do other stuff (make lunch, snooze, set up the tent etc.) it's irrelevant, and b) because my 10 yr old took care of water filtering. It definitely wasn't 1L/minute, at a guess it was 3-500ml/min. The key thing with gravity filters (I've used a few) is to ensure that there are no bubble at all in the system, from source to sink, in order to maximise the hydrostatic pressure in the system. That usually means lifting the sink above the source at some point to let the bubbles float out of the system.
