Check out Trendistic or What the Hashtag?! (you need to register for this one but it is free)
Tredistic has the option to switch between 24hrs, 7 days, 30 days, 90 or 180 days. However, after looking at Trendistic again it appears to only display the percentage not the actual number of tweets. What the Hashtag splits tweets in to days over a 7 day period, displaying the number of tweets on the graph.
I have been looking at Twitter API documentation and it seems to me that there wouldn't really be a 100% reliable way to get all of the mentions of a hashtag.
These are my observations after reading the API docs:
- History is only available for 7 days.
- When using the Search API you can specify dates as parameters but not times. I am presuming that the API would convert 09/01/2011 to 09/01/2011 00:00:00 but I can't be sure
- Not all tweets are available via the API. Presumably because some of them will be protected.
- You can only return a certain number of tweets with API calls. So if you have a particularly successful hashtag which spread across Twitter you wouldn't be able to retrieve the actual number of tweets anyway.