I can tell you in about 2 week :-) I am just finishing a machine based on those parts with a 50" X axis and a 48" y axis. The machine is "rolling" now - just finalizing the drive system. I was worried about flex when I first set it up, and there will definitely be a little bit, but it doesn't look like very much.
Note that I am creating a wood router, mostly for cutting operations though, so my tolerances to z-axis flex are fairly high. Note that my y axis gantry is pretty big/heavy too, as it has to carry the 20mm yaxis shafts, the gantry structure itself, and the z axis.