Was searching for the same thing - and found it - so for completeness for others -
Stream Analytics uses the default PowerBI data retention policy, which is 200,000 rows. After that, data is flushed out using FIFO.
If you want to remove data on your own, you can use the Power BI REST APIs to clear the rows of a table: http://docs.powerbi.apiary.io/#reference/datasets/table-rows/clear-the-rows-in-a-table