You can use the COLX Staking calculator here:
https://calculator.colossusxt.io/staking

The formula:

T = the Total number of COLX currently staked. Current number is 2.24 billion (June 26th 2018)
P = your Personal number of COLX staked.
1440 = The number of blocks in a day
T / (P * 1440) = your average number of days per reward