I believe any price below $1000 is a good time to buy. I buy whenever I am able to afford it. So I don't exactly have a certain time or day that I'm buying bitcoin. Whenever you can is the best way to go especially if you're holding bitcoin for a longer term.
$1000 is way too much. If in any case it reaches above $600 I'd already have second thoughts about buying. Right now what I do is set a portion of my monthly budget to buying coins, like 5%. It helps to save up.