10 am the previous day from the stn of origin.
For e.g. A train leaves from origin on Wed at 8:25 AM then the tatkal will open on tuesday 10 AM but if the train leaves the origin on Wed 11:55 pm also then the tatkal will open for it on Tuesday 10 am.
I suppose you will get a clear idea with this. If not I will be happy to explain you further.