In an interactive problem that I plan on setting, the interactor will have to do some nontrivial amount of work (say ~500ms). This only happens once, at the start of the interaction. On the one hand, 500ms isn't that long, but on the other hand... it's not an insignificant amount either, and could feasibly make or break whether a solution is AC or TLE.

Does anyone know how the Time Limit of interactive problems is computed on Polygon/Codeforces?