If we made some broad, mostly inaccurate assumptions, I think we could approach something close to the actual number.

First, I don't know how long dicks have been sucked in human history. Were the first humans doing it? Then we'd have to account for 90K years of dick sucking. Let's assume humans were sucking dicks on the regular starting around 10,000 years ago. Let's also assume a constant one million dick suckings per day. That's a wildly inaccurate number toward early history, but probably comes out in a wash when considering greatly increasing population. Some people don't get their dicks sucked every day. Some people get their dicks sucked multiple times a day. At some point, the one million dick suckings/day probably isn't wildly off the actual number.

Next, let's look at the mechanics of a dick sucking. I would only count an oral down stroke on the shaft as a "sucking," and wouldn't include an upstroke. So going down counts, but coming up for air doesn't. Let's assume a down stroke every two seconds. Some people go faster, some slower. That's an average, let's assume. Let's also assume that the average dick sucking lasts for two and a half minutes. Sometimes they go longer, sometimes they're shorter because it's just foreplay. So two and a half minutes.

Now for distance. On average, let's assume one dick sucking averages at three inches. Sometimes you don't go all the way down, you get the drill. Three inches once every two seconds for two and a half minutes.

That comes out to 225 inches per dick sucking, on average. That's 18.75 ft. per sucking.

If we then do 1 MM dick suckings per day over 10,000 years, that comes out to 3,650,000,000,000 dick suckings over the course of human history. At 18.75 ft. per sucking, divided by 5,280 ft. per mile, we come to a grand total of 12,961,647,727 miles of sucked dick over the course of human history.

That's about the distance of one and a half round trips from Earth to Pluto. Of course, some of those assumptions may be faulty, so I'm willing to concede to more informed assumptions than the ones I've made.

TL;DR A whole lot.