How long would a photon take to reach us if it's emitted from a galaxy receeding at c? A galaxy at the boundary of the Hubble sphere is receeding from us at the speed of light, right? If it emits a photon now, how long will it take to reach us?
Actually, this is a perfectly good question if interpreted in a reasonable way, i.e., interpreted as asking how long will light take to get to us if emitted at comoving age 13.8 billion years from a galaxy whose comoving distance is currently increasing from us at rate c (and the Hubble law with H=70 tells us that comoving distance is 4.3 Gpc or 14 billion LY).  That is not an infinite time, as that is not the definition of the Hubble sphere.  

The answer to how long it would take, less than infinity, depends on the cosmology model, and is a little hard to calculate because cosmology calculators (such as http://www.astro.ucla.edu/~wright/DlttCalc.html) tend to focus on light reaching us now-- as that's what we see.  For example, that calculator gives that a galaxy that is receding from us at rate c now would have had to emit light 9.25 billion years ago for us to be seeing it now, but that's not what was asked.

As mentioned, it is not easy to calculate the time it would take light emitted at the Hubble sphere now to get to us, but it is not infinite.  The Hubble sphere would only be the edge of what we could ever see if the Hubble constant were constant with time (as in an accelerating universe completely dominated by a cosmological constant), but even though the expansion does seem to be accelerating, it is not yet completely dominated by a cosmological constant (assuming that is what is causing the acceleration).  Hence, we would indeed eventually see the light emitted at the Hubble sphere now, but it might take a very long time, perhaps 50-100 billion years but that's just a guess without doing a real calculation with a full cosmological model.