Sitecore xDB - How to get count of interactions

If you need to rebuild your Path Analyzer maps, Sitecore have a knowledge base article on how to do it here - https://kb.sitecore.net/articles/766858

In the FAQ section it gives some info on how long you should expect it to take

What it doesn't tell you though is how you find out how many interactions your site has to process. The data is stored in analytics collection in your Mongo DB, so go there an then run...