I'm building UXWizz, which is on the same stack, and in my experience on a single server you can track up to 10-15 million sessions total (~1M sessions per month, with 1 year retention). To scale, for now I made it so you can have one database per domain, so the data is sharded by domain (if you are tracking multiple domains).

The biggest bottleneck is querying a large dataset, the storing of data seems to work pretty well. You could of course archive the data and automatically create stats using cron jobs, but this leads to a lot less filtering options and not so fresh data.