Essentially, they behave as if they've spent the last (however long you were offline) without sending any packets. If their lease is still valid, they should still be able to use the same IP address; i.e., they shouldn't need to re-acquire one. That's not always the case, so they also do a DHCP request along with that.
I have a late 2012 13". It usually takes ~8 seconds to connect to an AP. Seems to depend on the AP and anecdotally it's very sensitive to the wireless channel. Maybe I just have a bad card or something.
It's fast on Apple products because they 'cheat', and re-use the last-given DHCP address.
http://cafbit.com/entry/rapid_dhcp_or_how_do (and the follow up, http://cafbit.com/entry/rapid_dhcp_redux)