The level of detection avoidance that can be accomplished varies from network to network. When performing the test, keep in mind that, in this day and age, resources are usually very limited and administrators are overworked and underappreciated. Focus on bypassing the automated detection methodologies, and you are unlikely to be found by an active and eager admin unless your traffic and behavior patterns are drastically different from those of the average power user. When sniffing traffic and looking at network connections and activity, you should be able to get an idea of what is considered normal traffic on the network.
When performing scans, it may be a good idea to use multiple sources to originate the scan from. This is more likely to be possible in large networks, after a few people have clicked the links to your social engineering campaign page. Once you have several machines under your control, it is not advisable to scan from a...