It really depends on how you count it. Strictly counting the number of times my penis crosses the threshold to a strip club divided by the number of times it winds up in the mouth or vagina of a stripper while in the building, it would be somewhat less than 50%. I'm not sure if I'd get down to 10% involving extras, but 20% isn't unreasonable using that methodology.
But in a more generic sense, it's probably a bit over half the time involves extras. For example, often times I'll hit up two clubs on the same day, I might partake in some extras at one but not the other. If that counts as 50% of my visits, I'll be way below most of my visits involving extras. But if that counts as one visit I might be closer to 50%. If you count developing relationships for future OTC as involving extras, that would probably put me closer to 80% of my trips, since most of the time I'm at least looking for an OTC girl on my visit.
It changes over time too. I went a long time with zero interest in extras whatsoever, I had a stretch where that was basically the only reason I went, and I've spent a good amount of time where I'll partake or not depending on my mood, what's available, who I'm with, etc.