EMA crossover time frames and the ultimate question- When to put some cash to work?

Been watching two-line EMA on IVV and have been playing with 10-day up to 3-months and really not seeing a good firm crossover with which to determine it's a good time to put some cash to equities. Not a day-trade, just a buy and hold (+/-8yr time horizon). So kind of 'timing' the markets with all the weirdness going on and figured 2 line EMA would suffice to take some guess-work out of it. Looking for feedback on what time frame, smoothing, pricing frequency you use. (That and which charting site do you use if you're doing the same) (Bollinger overlays seem so far away on both up and down too)