Bitcoin difficulty adjusts every 2016 blocks. A block is mined about every 10 minutes. This means difficulty adjusts about every 2 weeks. There are about 4 weeks in a month.
There are ~3 difficulty changes per month due to the speed the network is growing
Bitcoin difficulty is adjusting about +1% per adjustment
Difficulty has been going up a *lot* more than 1% - last few adjustments:
2014-01-06 03:07:50 AM 1418481395
2014-01-01 09:30:40 PM 1180923195
2013-12-21 03:55:20 AM 908350862
2013-12-10 08:07:30 AM 707408283
2013-11-27 06:55:30 AM 609482679
2013-11-16 05:05:10 AM 510929738
2013-11-05 07:30:09 PM 390928787