Hi ninah - it depends what you mean by the 'spark' - there is sexual attraction, which is more common than what I think of as the spark, which is like a kismet thing. Fancying someone, especially when there is booze involved, probably happens to me about 1 in 3 dates, maybe 1 in 4.
Being somewhat cynical about it, and being a bloke (and knowing I might get flamed here) - knowing I could probably get them into bed if I really tried happens about one in 2 dates. But one night stands bore me to be honest, I'm not just out for a quickie so it's a passing point of interest unless there is a lot more. Sexual attraction and knowing I could get them into bed for a one time thing isn't necessarily the same thing. One is really wanting to get them into bed, the other is just knowing I could.
Then there is being mates with someone, getting on well with them enough to think - yeah, I'd really like to hang around with you again but I don't want to go to bed with you.
The spark, however, is the feeling - why oh why didnt I meet you earlier? Having stuff in common, bouncing thoughts off each other, finishing each others jokes and stuff like that, plus the sexual attraction - the Amazing Vanishing Historian felt like the Spark. For me the Spark is more than knowing I could, or wanting I could, it's wanting to, but not knowing if I could because she's a bit too special for me, maybe. Wanting to be with someone and being honoured to be with her, plus the connection. So the Kismet/Spark thing is the Mates, plus the Sexual Attraction, plus maybe something else indefinable - knowing how lucky I am to be here with this person.
That's just me of course. But I've had a couple of beers now as I'm narked with ShoeGirl.