Dawn would have been starting her sophomore year at High School in Season 7. It seemed like they kinda made out it was her first, but it was more because she was starting at Sunnydale High more than anything.
In Season 7 she was in the same year that Buffy, Xander & Willow were in Season 1.
As for Season 8, the time frame gap has purposefully been left vague. Seen as the "comic season" is gonna be a lot longer than a TV season in real time. A TV season would be around September to May for example, where as Season 8 started in April 2007 and isn't gonna finish till late 2010/early 2011 (our time)
With the army and gear that Team Buffy has amassed I'd put it at around 1-3 years after 'Chosen'