The last one was from 511 to 609.000.000 It's increasing for about 20% with every change, for the last 3-4 months.
Here are the actual numbers for the last few months:
Nov 17 2013 609,482,680 19.29% 4,362,847 GH/s
Nov 05 2013 510,929,738 30.70% 3,657,378 GH/s
Oct 26 2013 390,928,788 46.02% 2,798,377 GH/s
Oct 16 2013 267,731,249 41.45% 1,916,495 GH/s
Oct 06 2013 189,281,249 27.19% 1,354,928 GH/s
Sep 25 2013 148,819,200 32.13% 1,065,289 GH/s
Sep 14 2013 112,628,549 29.56% 806,227 GH/s
Sep 04 2013 86,933,018 32.22% 622,291 GH/s
Aug 24 2013 65,750,060 29.40% 470,657 GH/s
Aug 13 2013 50,810,339 35.88% 363,715 GH/s
Aug 03 2013 37,392,766 19.63% 267,668 GH/s
Jul 22 2013 31,256,961 19.47% 223,746 GH/s
Jul 11 2013 26,162,876 22.63% 187,281 GH/s
Jun 29 2013 21,335,329 10.32% 152,724 GH/s