As the others have posted, the dimpling can vary quite a bit. Since going to the no-sub method, it is not uncommon for my eggs to dimple very little or even not at all. I have a clutch at day 51 (about 4 days from pipping, by my average), with only 1 egg showing any dimpling whatsoever. Last year a clutch from the same female hatched without dimpling at all. The eggs seriously looked the same on day 55 as they did on day 1.
So here is the observation - Even though nearly all of my eggs stay "fully inflated", I do have one female whose eggs dimple quite early every year. They begin to dimply just slightly around 2 to 3 weeks in, then stay the same for the duration. I'm thinking this effect may have something to do with mom. I don't know if it has anything to do with the calcification of the egg, the structure of the membranes, the volume of fluids within, or what. Interesting really, but not that important...As long as they're not flattening out on you, six weeks in some slight dimpling if just fine....
Joe Pociask