1x R-Gacha gives 0:50 or better
2x R-Gacha give 1:00 or better
3x R-Gacha / 1x Ex-Wild give 1:30 or better
5x R-Gacha / 2x Ex-Wild give 2:00 or better
7x R-Gacha / 3x Ex wild give 2:30 or better
10x R-Gacha / 4x Ex wild give 3:00 or better

Chance of getting better wating times is only around 5% I think.
I only done 1x R-Gacha and 3x R-Gacha many times.
I donated more than 3x G-Gacha a few times but didnt get anything to make it worth while.
I have tried to donate less than 1x R-Gacha but so far I have not got any limmed girls and found it useless too.