In the last national election, how many days did it take for the EMB to announce certified results?

Representation of the People Act 1983

44(1)The returning officer shall make arrangements for counting the votes in the presence of the counting agents as soon as practicable after the close of the poll, and shall give to the counting agents notice in writing of the time and place at which he will begin to count the votes. […]

The administration of the June 2017 UK general election

2.74 The closest result at the June 2017 general election was in the constituency of North East Fife, which was decided by two votes after three formal recounts. The longest count was for the constituency of Kensington, which was decided by 20 votes after three formal recounts which continued throughout the night of polling day and during the following day. The result was declared at 9pm on Friday 9 June.

In the UK, election results are estblished by each local returning officer. During the latest elections in 2017, it took one day for results to be established in all the constituencies.