You would need to have some idea how relapse times are distributed within the interval.
For example, if they are exponentially distributed, then the mean of a moderate number of relapse times is roughly 17% of the way from the minimum to the maximum.
Here is a simulation for 200 subjects; results for 100 and were about 19% and results for 500 were about the 15%.
set.seed(2021)
m = 10^5; v = a = w = numeric(m)
for (i in 1:m) {
x = rexp(200)
v[i] = min(x); a[i] = mean(x); w[i] = max(x)
}
mean(v); mean(a); mean(w)
[1] 0.00499292
[1] 0.9999175
[1] 5.875548
(mean(a)-mean(v))/(mean(w)-mean(v))
[1] 0.1694771
Note: For a uniform and normal relapse distributions (both symmetrical)distribution, results were about 50%.