At what point does crosstalk on I2C become a problem?

If I have a pair of I2C traces (SDA and SCL) on a 1 oz board, each being 8mil wide and separated by an 8mil gap, how long would the traces have to be before crosstalk would become an issue?

Edit:

Apologies for not making this clear before, but I was asking from a theoretical point of view, and with everything else considered equal.

I was checking the rise times on an I2C bus, and the crosstalk spikes were larger than I would have expected (300-400mV). After reading some of the replies, I measured the fall time - it's about 15nS, and from my calculation I must have about 80pF of capacitance on the bus.