Can't say about Aerocity, but in a private car currently it should not take more than 30 minutes at that time of the day, even less if we are talking about a Saturday or Sunday.
You say you will be travelling next year. It is good to know that on Jan 29th, the new Barajas terminal (T4) will be officially opened. It is new, big, wonderful... and far North from the other three terminals and will not be served by metro or trains by the moment. The arrival of metro and Cercanias (local trains' system) is expected by 2008. This new terminal will mainly host all Iberia flights and those of their partners in the OneWorld alliance (Finnair, British Airways, American Airlines,...). Some other small companies will be located there, too.
The other two big alliances, Star Alliance (Spanish partner, Spanair; American ones, United and US Airways ), and Sky Team (Spanish partner next to enter, AirEuropa; American ones, Delta and Continental) will stay in the former terminals, Star at T1, and Sky at T2 & 3.
I am mentioning this, because if you arrive at T4 next year, the estimated time to reach your lodgement in downtown Madrid can be a bit longer.