There are only two ways in Marvel Puzzle Quest to receive a bonus when purchasing in-game currency. One is known as the 'first-time buy' bonus, where you will receive a bonus if it is the first purchase of that type that you have made that day. The second, occasionally, a sale will premiere giving you a 30% increase on all in-game currency purchases. As for the time frame, there isn't a set one, however the duration of the sale is usually a week.