How to impose this heat flux boundary condition?
 

Dear friends,

if the heat flux was like that:

from t = 0.0 up to 180.0 [s] => q" = 0.0 [W m^-2];
from t = 181.0 up to 1,079.0 [s] => q" = 250.0 [W m^-2] (=>constant);
from t = 1,080.0 up to 1,260.0 [s] => q" = 0.0 [W m^-2].

best regards,
Rogerio.:D

I would use a 1D interpolation function.