If you manage your own investments then you need to rebalance your portfolio every 3 months. One investment should never be more than 20% of your portfolio.
At first glance this looks somewhat diversified, but upon further inspection you will see the problems that need to be fixed. Anadarko, BP, and Exxon are all Energy plays, this represents 33% of this portfolio, Some of these positions need to be sold off and invested into another sector, maybe retail or food. Also you see Microsoft and Yahoo, these are both tech companies and the point could effectively be argued that they yahoo is more of a media company then a tech company, but it follows the trends of the tech sector more than the trends of the media sector. I would personally pare back my positions in Microsoft or Yahoo to make a combined total of 20%.
This needs to be done every 3 months simply because some investments will lag while others will prosper, eventually your portfolio will be lop-sided and no one wants a lop-sided portfolio.