Hey Dock.......it will depend on a few things as to how long it actually lasts.....
Two briq. spacing is not too much, so the heat would most likely be pretty low.....people use two with one on top & even two, two on that & one on top of those to build up an area of fuel.....
Then it becomes a matter of how much air is applied to the fuel pack & if you're using wood chunks (or chips) ...how much of those are added in.....
A standard setup for me might be, under medium outside air temperature (yes, if it's 105f where you live that day, it will have some effect!!!)......two briquettes with one on top, 5 or 6 wood chunks spaced around the ring, ring maybe 7/8's of the way around, chips of lump charcoal sprinkled over the pile all along,10 briq's to lite the chain, bottom vents 1/2 or a little less open, top vent all open, temp will be at approx. 240~260f AT GRATE LEVEL.....for about 9 to 10 hours, maybe more if the BBQ gods say so.......
I always just stack most of the way around.....if that much isn't needed, then just break the chain & save the unburnt coals.....
It will vary always a little, depending on little things like if your charcoal has taken on a little moisture, what brand as some burn quicker than others, what wood you might use as that's an additional fuel & if it flares up & burns hot, it will move the chain a little faster.....some folks will pinch the vents pretty far down & even pinch down the top vent to run less total air thru.....
I like to let the air / smoke exhaust freely so there's no stalled, stale smoke dwelling in the cooker....but as you get the hang of it, that can be a temp control tool to use a little......
Here's a few of my links for long & low cooks that explain some setups that get pretty even temps....
http://weberkettleclub.com/forums/other-recipes/something-different/http://weberkettleclub.com/forums/grilling-bbqing/ok-finally-brisket/http://weberkettleclub.com/forums/food-pr0n/bob's-beef-o-rama/msg20382/#msg20382http://weberkettleclub.com/forums/grilling-bbqing/look-ma-no-therms/msg114445/#msg114445Others put up good pics & descriptions of setup also.....most likely they'll be in "grilling & BBQ" or the "Food Pr0on" sections & be about the longer cooks like brisket & pork butt..... also works pretty good for ribs, with a few adjustments.....
*** There's a nice setup from rivercityjeff right there.....with some pertinent info, too!!!