In a no protection situation, can STD risk be minimized by time?

If AIDS (or other STD's) are contracted through exchange of fluids; and neither the girl or guy have open sores or exchange seminal fluids during oral - can this disease (or others) be transferred by saliva or (during intercourse) vaginal juice simply “soaking into” the thin skin of the penis?
If this is the case, does that mean that the amount of time receiving oral or having sex will increase the chance of contracting an STD?
i.e. quick oral and/or quick sex will decrease the chance of carriers “saliva/juices” to penetrate and infect?
I realize this is all “high risk” but does time affect chances?

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Answer:
You're probably right, with most infections bacterial/viral load is a factor in pathogenesis (infectivity). However, this obviously isn't a reliable measure, and some viruses are exceptions to the rule, and require only a small amount of organism to start the infection off.