For me it doesn't matter, but it would probably better to be credited the rakeback every 30 days, during the end of the month to have something to expect, and welcome in our balance every month. I believe users would have a tendency to manually claim, then gamble with them as soon as they can if given that choice, which might also make them feel less willing to play next time if they also lose the rakeback.
Cashback is usually have this kind of duration since its the percentage on the amount of loss done by the player. Rakeback is just part of the house edge and thats is just so small to be hold for 30 days.
@Sujonali, is there plan to have another cashback event but this time for all players including low bankroll. The last event focus only on whales but I remember last time that Blackjack.fun has some deposit bonus event based on tier that varies wagering requirements. Maybe this time the new cashback event will have a tier that varies the percentage based on the user bankroll/loss value.