0.7 was first announced in early April and released in mid November, making for a cool 6 1/2 months of anticipation. Of course, 0.7 was revealed shortly after the final updates to version 0.6 and was probably the largest update so far in terms of expanding the scope of the game, so it's reasonable that there was still a lot of work to do at that point. 0.8 looks to be an even larger expansion of the game and required the development of entirely new systems like the procedural sector generator in addition to other new features that Alex hasn't even revealed to us yet, so I wouldn't expect it particularly soon. The good news is that he's been quietly plugging away at this one for the better part of a year already, so it's probably closer to completion than 0.7 was at its announcement unless implementing something proves to be more complicated than he expected (which is entirely possible given the stated scope of the additions).
I'd guess 2-3 months, though not with any real authority, and if it's sooner than 2 months I'll eat a sock.