Here's a rough and ready way (I'll leave you to implement Heun's method):
f = @(H) -0.06*sqrt(H);
H = 4;
t = 0;
dt = 0.1; % choose your own value
while H>0
t = t+dt;
% Use Heun's method to get H at t+dt
end
disp(t) % time to drain
Note that, because of the square root, the last step is likely to result in a complex value for the final value of H.
