This is up to the 1922 Committee and they haven't announced a time. Most likely it will be in the afternoon as the deadline for returning ballot papers is 5 pm the day before, so they'll want time to count papers that arrive at the last minute and check the totals.
Last time there was a ballot of the whole party membership was in 2005 and I can't track down what time that result was announced.
About the only thing I can say is who will announce it - Cheryl Gillan MP, Chairman of the 1922 Committee.
What is more certain is when the new leader becomes PM. Theresa May is planning to answer PMQs for the last time on Wednesday lunchtime. I expect PMQs to consist of lots of fawning and "sorry to see you go", at least from the Conservative side. Unlikely from Jeremy Corbyn as he has been most ungracious about her departure so far. The she will visit the Queen to formally resign (not necessary but it's traditional). Then the new leader will visit the Palace to "kiss hands" upon appointment soon afterwards. The new leader is not PM until this is done.
BTW the 1922 Committee, if you didn't know, consists of all Conservative backbench MPs and under party rules, it conducts leadership elections.