What is the time period until when I can authorize my sell transactions?

Once you receive a notification from Upstox at the end of a trade (T) day you need to authorize your trades on CDSL by 

7:00 PM on the same day for stocks which are unavailable under Demat holdings.

12.00 PM next settlement day for T+2 delivery stocks which are available under Demat holdings.

8:00 PM on the same day for T+1 delivery stocks which are available under Demat holdings

This means if you place a sell trade on Monday, you can authorize it till:
Monday 7:00 PM for stocks which are unavailable under Demat holdings.

Tuesday 12:00 PM for T+2 delivery stocks which are available under Demat holdings.

Monday 12:00 PM for T+1 delivery stocks which are available under Demat holdings.