I am trying to override the default SqlConnection
timeout of 15 seconds and am getting an error saying that the
property or indexer cann
You can set the timeout value in the connection string, but after you've connected it's read-only. You can read more at http://msdn.microsoft.com/en-us/library/system.data.sqlclient.sqlconnection.connectiontimeout.aspx
As Anil implies, ConnectionTimeout may not be what you need; it controls how long the ADO driver will wait when establishing a new connection. Your usage seems to indicate a need to wait longer than normal for a particular SQL query to execute, and in that case Anil is exactly right; use CommandTimeout (which is R/W) to change the expected completion time for an individual SqlCommand.