Defragging partitions depends on your usage... The Windows partition may need to be defragged every 15-30 days... But other partitions may depend on how frequently you use it and what file system is used... Fat32 partitions are more fragmented by nature compared to NTFS partitions... Partitions with more common write operations are likely to be more fragmented...
So, you need to work out by analysing the partitions regularly to figure out how often you need to defrag your various partitions...
Arun