You should aim flush a fountain pen on average 4 times a year for most fountain pens with standard inks and everyday usage. You might see pen enthusiasts cleaning their pens every time they refill it.  This is not really necessary unless you are using special inks like glitter inks or thicker inks.  However, most users of fountain pens actually enjoy looking after their pens, it is not just a writing instrument for them, it is an enjoyable hobby aswell and cleaning their pen is part of the routine. You should also clean your pen when you change your ink colour aswell, for obvious reasons, otherwise the old ink colour will continue to bleed into the new ink colour.