Honestly, I don't really do it a lot since I time travel by the hour to keep the shops open but if I know somebody with good turnip prices, I'll time travel to Sunday morning and buy a bunch.
I do it if the selling price is high and/or I am needing more. My three active towns, listed in my signature, are not in need. But, I have two side towns, not ready for show, which I am developing. My more recent turnips activity, using more than one game console, focuses on those other towns. So, the time I strike is when I draw an impressive selling price.
I've actually never bought turnips once in my entire time of playing New Leaf. I'm talking like almost 2 years of playing. I'm planning on buying them soon, though. As I'm aiming for the gold turnip badge now.
I think the whole time I've played ACNL I have only bought turnips at max like 5 times... and two of the times I TT'd and destroyed them X.X I will eventually buy a lot of them to get the badge but I'm waiting until I get my money badge first.