I will only buy games that I can try first. Mostly, will they actually WORK on my Mac? Sometimes a game doesn't run, or doesn't run properly for me, despite meeting the posted system requirements. No way I want to waste my $$ on that! And others turn out to be not what I expected at all, but entirely not to my liking.
The time limit option is also fine, but rarely can I get a good enough sample in an hour to decide if I actually will enjoy more of it. Most of those I've tried, I don't end up buying. I opt instead for something I've played longer, that I'm sure I want to continue. An hour is barely enough to get the hang of the controls of many games (sometimes not enough even for that) and I just just don't enjoy it, so I don't buy.