The term buying  Dip is just used when there's a decline in price, it could be small or long doesn't matter the period but longer period indicating a decline in price are likely to bring about longer Dip, yet a long period (Time) could still yield a small dip as it all depends on the market  ..
 Thus, we use the phrase "Buy the Dip" when we believe there's going to be a reversal after a decline in price doesn't  matter the period it will take to reverse but we mostly belive it's likely going to be a short one(short here could be months.. depends on how you define it ), even though some might take longer than expected.
At the same time, there are trends, Uptrend ~ Downtrend and each of this trend can carry as mush as possible Dips you could imagine, a decline could result in a trend reversal from uptrend to downtrend , giving opportunities to buy as many as possible dip you could scoop out of the trend BUT in most cases, you can predict the period a decline will last.. there could even be a  very short decline and immediately followed by a full pump and that brings us to why we shouldn't wait for a specific period to get our Dip not especially as someone in his/her accumulation stage.
DCA is more like buying a DiP so far the price Is below the ATH although the lower the price, the longer the Dip gets( which means bigger  opportunity) yet DCA covers it, so far it's done the right way , simply by just maintaining consistency doesn't necessarily have to be a fixed price but having a range to meet up, is much more better  .. Whichever works for you (range or fixed) just make sure you are consistent in your buying plan.