I think 2 - 3 days is the 'sweet spot' but blockchain.info doesn't allow me to go back more than 250 blocks in their API..
(I asked them in their thread, no response..)
Other site I can use is http://blockorigin.pfoe.be/blocklist.php but they don't include timestamps AND some mined blocks by ASICMINER show up on their list as unknown...
If someone knows a site with an API which goes back than 500 blocks (or more) then please let me know
I have blocks and (translated from hex) coinbases for the last couple weeks. They update in realtime, but I don't have anywhere to put them for real time access.
If you use R I'd be happy to share with you the code that queries bitcoind, records blocks, blocktimes and translates the coinbase (although running bitcoind from a hosted server may be a problem). Easy from there to generate nice charts automatically.
Otherwise you can get the coinbase for all blocks from blockexplorer.com, but you'd have to grep the hex value of the ASICMiner coinbase signature yourself.