The first thing I would do is seal that concrete floor, just incase. Murphy's law number two always applies. The easiest and probably cheapest sealer for concrete floors is paste wax, it will last around a year and with that applied you won't have to worry about nasty things seeping into the concrete.
I would change the straw when it started looking like it might need changing, eyes and noses are always the best indicators for this. The amount of time will be dependent on how many chooks you have, just keep an eye and nose open to how it looks, when it seems to be getting "full of poop" then it is time to change it out.
If you want to go a little deeper with the litter, and so add more time between clean outs.
You can put a "ledger" board along the bottom of the door (will require you to step over it so you don't trip) this will allow you to use a deeper litter method. You can use a silicon sealant on the edge you place to the concrete floor and one screw on each end into the door jamb to hold it in place. Use at least a 1 x 4 and perhaps a 1 x 6 if you are comfortable stepping over something that high.