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
Depending on the processing power of your server, your XDB configuration, this could take from minutes to hours. Most of the time is spent on bringing over and de-serializing XDB interaction data on the Processing Server.
For the reference, it takes approximately 2 hours on XDB with 7M interactions, running on a single server.
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…