Can virus resistance be acquired through generational exposure?
Perhaps the answer is in natural selection. Over many generations, if these plants are constantly exposed to the same virus, one of them may obtain a mutation or something similar that allows it to be resistant, thus that plant will have higher survival and growth rate and thus more seeds. 

The current plant itself may also adapt to the presence of this virus (perhaps in growth patterns, etc.) over a long period of time.

As for the time for the genetic resistance to take place, it is almost impossible to say as it is almost completely random (and would also depend on what other plants you may pollinate it with as an already resistant plant would, of course, cancel the need for a mutation).

I hope this helps.