Someone did the math on this but I can't remember the exact numbers
I think it was 30 days or voucher drops so 1 million divided by 30 is a little over 33,000 a day being dropped for vouchers.
Currently there is 94.4 million staked (which will most likely increase by a lot soon) so that would be 1 per every 2,848 SPS staked I believe? My math might be totally off on that though