For the FGT Championship, it should take you about 2 hours, depending how skilled you are & how fast you drive. For the Dream Car Championship, it should also take you about 2 hours or more, with also the differencing factors. Even though both events has the same times, the FGT Championship is the highest elapsed time throughout all the events, as each race is 20 laps.
I can't speak for the FGT, but the Dream Car Championship took me less than an hour with the X2010 S. Vettel. (Kinda got dizzy driving that thing on the Nürburgring 24hr course )