It seems odd that your radio would only stay on for a few minutes. However, if you push the start/stop button until your dash lights are on. Your radio should stay on indefinitely then. Or at least until the battery dies.
2006 E90 330i 6MT: TiAg|Black Leather|Poplar Trim|Premium Package|Cold Weather Package
2003 E46 325i 5MT: Retired - The beginning of my BMW obsession!