Lots of speculations have been made as to the time frame and location using clues from within the manga and animation.....but why not just ask the series creator if he gave it much thought or is the story just a mishmash of past and future technologies. I'm sure much Japanese writing has been done on this topic that hasn't been translated.....or perhaps, we Westerners are more concerned about this than we should be.
I always thought of it as a futuristic collapsed society type theme. They were forced to build walls and revert back to medieval society but they focused their technology on their ODM gear to combat the titans. There is reference to the past (our current real life time). Perhaps the biggest supporting evidence of this was episode 6 where Misaka's kidnappers are talking about her race and they state:
"she's exotic. what they used to call an Oriental.You know about all that right? back when there used to be different breeds of people, a bunch of them from the far east came scrambling here for safety. the old perverts in the capital really go for that sort of thing. this little beauty's going to fetch a mint at auction. it's great business. she's the last of her kind. all the rest of her people died out."
hope that helps. the year that it starts out on is 845 so the yearly calendar reset for some important reason, but it hasn't revealed a lot about the history yet (just that the titans appeared over 100 years ago.) As far as location goes it's safe to assume that it is probably German. accounting for the kidnapper's claim that there is currently only one race and we have names like: -Yaeger (german for hunter) -Arlert is old german anglo-saxon, and Armin's wiki page trivia section states that "The name 'Armin' may have multiple meanings, either from an old German word meaning "whole", or "Herman", meaning "soldier"". http://shingekinokyojin.wikia.com/wiki/Armin_Arlert -Erwin - Derived from the Germanic name Hariwini, composed of the elements hari "army" and win "friend".
Add it all up with a lot of blonde hair (Armin, Reiner, Nanaba, Christa, Annie, Erwin, etc) and it seems that they were leaning toward a German-oriented character base with names that are literal puns toward the fact that they are soldiers.