Whatever the market environment fits. During high volatility, the shorter the time-frame, the better, because sentiment changes faster and you can't have a strong opinion for the long run based on those fast changes. That is based on the traded instrument as well, through experience or historical testing you should know how an instrument behaves according to the overall market volatility. There are times when SPY makes a 1.5% move in either direction in the span of an hour - such are the trading conditions now. And there are times like late 2016 through 2017, where SPY was grinding with 0,30%-0,50% daily moves to the downside and those being "buy-the-dip" opportunities as well! I personally like to bring it down to the 1-minute time frame to try and execute a trade with whatever higher time horizon. If I already have a signal on a daily/weekly basis, I try and get a better execution on a 30, 15, 5 min, just to hold a losing trade for as little time as possible.