As far as I read it, you 'need' to do 4 training sessions a day. If you have two matches in a day, then it prompts you to do two sessions before each match. If you only have 1 match, then the prompt is for four sessions. The latter is a real PITA if you have a match at 23:30 in the evening and the next and only match the next day is at 10:34... stock up on Green Packs!!
I usually wait for 1/2 - 1 hour after the current match before doing the training sessions (one/two for the subs and one/two for all players), wait for the 6% every three hours and then see what the situation is just before the next match. In the above example I only needed to use 6 packs to get the starting XI up to at least 95% condition each - the subs I don't bother with.
PS: Having not chanced playing a match without doing the 'required' training sessions, I still don't know whether there is one, or what the penalty is in-match for not doing the training sessions...