Solr Index is being refreshed automatically at certain time interval. By default it is scheduled to re-index pending product changes at every one hour. However, you can change this interval by changing its schedule task by going to nopCommerce Administration Area then System -> Schedule Task -> Solr IDI process.
Posted in: Support
Have a look at nopAccelerate Demo Store with 80,000+ products with nopAccelerate Solr and CDN Plugin.