I have found a link (http://www.ukba.homeoffice.gov.uk/worki ... /earnings/) stating the following:
I am applying for Tier 1 now. Does that mean they will only consider my earnings for 48 hours?We only award points for hours of up to 48 hours per week, even if you work longer. For example, if your annual earnings are £25,000 for 60 hours of work per week, the same earnings for 48 hours of work per week would be £20,000. So you would be able to claim 10 points for your earnings.
Thanks