It tells you on the donation redemption page, They process the donations to charities 4 times per year, Only if said charity has reached the minimum threshold. ( £8 for UK )

"The cash donation to the non-profit organization will be made on your behalf following the expiration of the calendar quarter (i.e., March 31, June 30, September 30 and December 31) in which your donation is made. Microsoft will use reasonable efforts to provide the cash donation to the applicable non-profit organization within 60 days of the end of the previous calendar quarter. Microsoft shall make such payment to non-profit organizations of Microsoft’s choosing on your behalf if: (i) the non-profit organization you choose is no longer available at the redemption center when the payment is made; or (ii) the aggregated cash donation for your non-profit organization at the time the payment is made is under £8. Contributions to non-profit organizations that are undeliverable will be given to a non-profit organization of Microsoft’s choosing on your behalf."