you should change your needle with every new project or 8 hours of sewing time what ever comes first...and your machine should be cleaned (especially in the bobbin - throat area) every time you change your bobbin...and more often if working with flannels or other (linty fabrics)
follow your manual's instuctions for cleaning and oiling and your machine will last for generations...just use it like a work horse with no regular maintenence and you will be shopping for a new one in no time