Typically, you will only hear long conversations or exchanges during surveillance. On top of that, a lot of the surveillance is encrypted. During operations, they will usually make sure they are all encrypted.
Occasionally, an agent will forget to flip the switch or not have the key in their radio, so all will be in the clear. This is rare.
Expect to hear short blips of encrypted traffic. Also, most surveillance is done simplex. So, if you're not close enough, the scanner won't pick it up. I would say, within a mile radius, give or take a bit, should be close enough. There is no need to repeat surveillance traffic, usually.