Hello there fellow managers!

Being a big fan of football, and also playing it myself a long time ago, I decided to start playing top eleven and so far I have been enjoying this game very much.
I just have one question wich I cant seem to find any answer to from anywhere, so here it goes:
How long before your match starts can u still change tactics? Last time I tried to change them just before the match started, it was too late.
Im asking this because I sometimes have 2 games during the night/morning, and I would like to make some changes to the second match but I dont know when I have to wake up and do them.

Thanks in advance.