The Institute for Scientific Information (ISI) publishes yearly in the database Journal Citation Reports (JCR) the impact factor of scientific journals.
The impact factor of a journal is a "citation standard" to show how often articles from that journal are cited on average in a specific year. The impact factor shows how important a journal is in comparison to other journals from the same subject area.
"The impact factor of a journal is calculated by dividing the number of current year citations to the source items published in that journal during the previous two years." (http://thomsonreuters.com/products_services/science/free/essays/impact_factor/)
Thus the impact factor of a journal is calculated by taking the references to articles in the past two years.
This means that it takes two years for a journal to receive a new impact factor.
Besides the yearly impact factor you can also find other indicators for a journal: the 5- year impact factor, the Eigenfactor and the Article influence score.
When you look for a journal in Limo, you can by opening the Librilinksmenu find a link to the IF-info of that specific journal, if it is available.
Recently Project Counter offers you also the Journal Usage Factor (JUF) which is an indicator for the use of e-journals.