Two questions about the times of troubles:

1)How long were they (months, years)?

2) Mystra forsaw her own dead and she gave mortals (like Elminster) special powers, but when did she do this(the exact year would be very helpfull)?

It cant say much about your second question and the answer I got on your first comes from FR literature, no source books or anything. But according to Salvatore so was the time of troubles very brief. After playing ToB I wonder how the heck Bhaal had time to father all those offspring he must have been a real ladies man. How I interpreted Salvatores writings so was the time of troubles about 6 months long. Magic went awry and the Oblodra house in Menzoberranzan were the only drow with power for awhile (psionics) but they didnt have enough time to fully take advantage of the situation so it was no years as I had first thought.

Well, I don't really know much about the time of troubles myself, but you might want to try this link:
http://www.geocities.com/TimesSquare/Castle/2566/realmstime6.htm

The Time of Troubles begins at '1358/Year of Shadows /(1333 CR)'. I don't think there's any specific information on how many months it lasts, but it looks to be less than a full year. Hope this helps a little.

Thanks, that link helped a lot. It gave me some more insight in the years of the Forgotten Realms.