Viper has limits for free scanning, which works out at roughly 2 documents per day. The free scanning limit will reset 24 hours after your last scan.
The Viper scanning limits are based around the number of searches the software makes in your document. Viper takes your document, breaks it up, and then searches parts of it against online sources. How many searches it makes will vary depending on the length of your document (i.e. how many words are in the document) and how much content matches other sources.
If you hit your limit, any unscanned documents will be queued for 24 hours.