45 days from the end of the month where it got added...So upto 75 days, if you get the cashback on say May 1st & 45-days from May 31st would be around July 15th, which works out to 75 days.. But the yeah if you got the cashback on 31st May.. then it is still July 15th.. i.e 45 days.
And its usable 10% at a time. So unless you keep adding/getting cashbacks. A 50 rupee cashback will get exhausted over around 20-25 transactions...