I've soaked them under pressure for several days. Don't remember how much, maybe 50-60# or so. 11/32 cedar shafts gained 200 grains out of the tube, compared to about 50 grains if I just soaked them for four or five days. However, they then bled for the next two weeks, and I had to wipe them down daily. Finally dried out after about two weeks and I ended up with a net gain averaging 75 grains per shaft.
I no longer soak my shafts to add weight. When I was doing it, a 160 grain head was about as heavy as one could find. Now, it's pretty easy to find glue on heads to 250 grains and more. So, one can get the same weight arrow just by going to a heavier head, adding woody weights or an internal metal footing, and get the additional advantage of more FOC.
Never tried soaking hardwoods, but did try it on Doug fir. 20 grains per shaft average is the best I ever got soaking them in Watco oil. Just doesn't suck it up like cedar. I expect hardwoods would be even less likely to soak up the oil..