Exponential decay is exponential growth in reverse, so for example, if the R were 2 during the growth phase, and the R is now 0.5, you would expect it to take t

Exponential decay is exponential growth in reverse, so for example, if the R were 2 during the growth phase, and the R is now 0.5, you would expect it to take the same amount of time to decrease as it did to build up. If the R were reduced to 0.99 it would take years to go away. To give you an idea, after 10 rounds of people getting infected, spreading it onward, and recovering, the number of cases would only have decayed to around 90% of its peak value (0.99 ^ 10 = 0.904)