The amount is changing constantly because it depends on the price of 1 CSPR.

Besides that you can easily check how much rewards you've earned for each delegator and multiply by 12.

E. g.:

You earn 10 CSPR per era -> 12x 10 CSPR = 120 CSPR per day

Rewards are earned per era and 1 era is defined as 2h so 1 day has 12 eras (24h / 2h = 12)