Your user dashboard will give you the exact date and time, but I believe it runs out at the exact time (my last one ran out an hour later than it was activated but now that I think about it, I'm pretty sure daylight savings was what threw that off rather than it being intentional)
I think there's an hour's grace period in there but if you bought your GA at March 27th, 2019 at 11:02PM EST, it should run out exactly at April 27th, 2019 at 11:02 PM EST if it's a month one.

This might help in your decision if you haven't made it already: what I usually do on the day I purchase a GA is do all the Wizard quests first, and then activate the GA and do the last five. That way I get the bonus tokens twice in the same day!
Just something else to consider for GA timing. :)
haha yes that's what i do too. The only concern is sometimes it's hard to fit 15 of each quest all at once so if it runs according to exact time, i'd like to start at 11pm so i can get in those last 5 each after the account is activated, plus also have until the very end of the last day to get them all in too. And yes indeed that extra 50 wiz tokens are a nice bonus!