If I stop adding new cards, how fast will my reviews go down?

This depends on how many cards you get correct. If you consistently get 80% correct, the amount of cards after the first week will be about 75% of the previous week. After a month your workload will be about 25% and 3 months later, it’ll average around 1-2 percent of the original workload. Note that, a small increase or decrease in your correct percentage will drastically change these estimates.