Quick answer? It depends.

Anyone who needs is looking to create a social network post-haste should visit Ning. Ning allows you to create a social network for free, in less than a few minutes. For small groups or clubs, this may be all you need.

Most companies, however, are looking for a more robust social networking tool that they can install behind their firewall, and possibly integrate with existing systems or application. This will require a good amount of custom development, and is more difficult to determine a timeframe. This is why vendor selection is such an important part of the process.  A good vendor will be able to ask the right questions to determine what features you really need for the release and which features can be added in a second phase, etc... From this, they should be able to give you a good idea of how long the process will take.

Here is a rough idea of what to expect: 

Open-Source Solution: 4 - 6 weeks

Open-Source (Custom Development): 6 - 12 weeks

Custom Solution: 12 + weeks