Sorta depends where you are in the world and when your playtime is. It is quite possible to lose 2 days of playtime in a 24 hour period.
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So it started today at 2am which means you could play up until 2am. You're missing the tail end of that day at a point when "most people will be sleeping", not a whole second day of access.
Assuming you have the same period of time free for gaming each day, a 24-hour maintenance will either fully cover one of your play periods or will take half from one, half from the other, adding up to a whole of one period lost.
Yeah, maintenance for major patches (5.4 in this case) is usually 24 hours but sometimes ends an hour or two early.
So usually you will not be able to play on the day before a major patch, usually Monday for the .x patches though the exact time and day will vary depending on your time zone.
Sometimes this interferes with completing limited time or seasonal
events, so be forewarned!
Note you can usually download the version update before server maintenance actually ends.