The results of networking takes place over time. You should measure progress over a time segment based on your sales cycle. For example, if it takes two months to convert a network contact into a customer, than you should measure your ROI over a two month time period.
In addition, networking is only a component of how you build your sales pipeline and is used in conjunction with other sales and marketing tactics. Networking should be embedded into your process. This tool will help you calculate the extent that networking adds value in the sales process.