That's a question that involves many different variables. The 3x leveraged ETF's almost always gap one way or the other during pre market trading. I typically don't hold overnight unless there is data coming out the next day that I think will make for some good volatility. For example, I took a position in FAZ last Monday when the market was up 200 points and I held it the rest of the week knowing the market would probably be relatively flat ahead of the jobs numbers on Friday. Sure enough, FAZ traded in a 'calm' range between 13-13.30 and then Friday hit a high of 14.04 off the bad jobs number so I locked in profit. I typically NEVER hold them over the weekend because of all the different factors that may come into play doing so. Another point of note with FAZ/FAS and the weekend is that year to date, Monday's have been the best day out of the week for the stock market and Tuesdays the worse. So typically I buy FAZ Monday and sell Tuesday.
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