Once you've done that, you should start to see sessions in your account within seconds (assuming you have any active users on your site or app!). The newest sessions will show up at the top of the user list.
These most recent and live sessions are not necessarily fully indexed for search. Indexing is triggered by user-directed page transitions. As a result, for long-running single pages – or single-page apps in particular – you may observe a 5-30 minute delay before new sessions are indexed and returned in your search results.