45 minutes.
That seems stupid, but you can usually tell the major flaws and strengths after that long.
I set up my side based on the theories I like in the T&TT Frameworks thread (usually the rule of two) and based on my own experience of what might work. Then I watch a full half on either extended or full highlights, tweaking as I go (stopping gaping holes, stopping poor players playing through balls etc.)
By the end of the first half, you can tell if you're well off or not.
After 3 games against different teams you can tell if the basis of the tactic is right.
From then, I stick with it unless my results completely bomb out (i.e. I'm playing worse than what the board expected of me at the beginning of the year). I will change settings, play different styles away, at home, against good opposition etc, but my basic foundation tactic remains the same for 12 months.
Then, when the new batch of youngsters/transfers come in in July, it's time to start afresh on the same course - tweaking and changing.
I change my tactic drastically (formation, fundamental style of play) probably once every 24 months, but only when obviously required.