When I asked my mother she said that she doesn't have it written somewhere. Most probably before the half of the hour but I don't know exactly when. She said it was approximately 6.30 pm ... so I believe it's somewhere between 20-30 minutes.
Being 20-30 minutes off the mark is a huge huge difference in astrology. It often means the difference between being on the mark and having your chart seem like somebody you don't know. In your case it could easily send your ASC possibly into LEO, which makes an enormous difference in interpretation of the chart. with that much difference, I would not be willing to give the most important event in your life which your career a go, knowing that I could easily be very very wrong. I consider the maximum time off of being about 6 minutes; more than that means it could be actually more than 12 minutes off from reality. In astrology 12 minutes is a substantial margin. This is my opinion, of course.
It is possible to rectify the chart using 10-15 major event in your life, but if you don't have major events such as deaths, graduations, births, marriages, etc, then its just kind of hard to get something accurate for awhile.