A tip I picked up was to change your needle every time you have wound 10 bobbins! Another was an average of every 8 sewing hours, me I change them for every time I quilt a new project or when I feel that I need to :twisted:
To me there is no set rule, you can after a time feel and hear when you need to change them also where I buy needles they are only £10 for 100 needles, Organ brand which are great so I can be rather frivolous with them :-P
A thing to remember, as my Machine supplier told me, up to70% of machine problems are caused by needle problems :shock: