I believe you speak of some species of Formica spp., such as Formica polyctena (nest example)
You should mind that (i) ants are not particularly noisy insects; (ii) ant nests are 3-dimensional structures within which ants are heterogeneously distributed. So considering a microphone somewhere on the nest where you happened to see more ants, mostly you will be picking up noises from foragers stumbling into debris while moving around. And that only when that region of the nest happens to be most active, which was your original concern. In case I am wrong and you're interested in some smaller ant species (e.g. Solenopsis fugax or Lasius flavus) then you will probably record few if any significant sounds resulting from ants' activities.
Thus my first advice if you're focusing on foragers moving around the nest surface is that you record them in the afternoon of your hottest days, particularly if they're refurbishing their nest or upon disturbance. If you leave the microphone for extended periods try to find out where most foragers are coming out from (and pray they'll not damage the equipment) and leave it there in the afternoon of a dry, hot day.
Now, I am not sure about your ultimate goal, but in case you want to actually hear the ants instead of their stumbling around, then you should focus on having a tiny (protected) microphone inserted into a nest chamber. It is hard to say when the insides of a nest of unknown species will be particularly active, but usually during a fair day, under the sunlight exposure following a wet night. Finding the best region to record ants stridulating inside their nest will depend on finding the best nest location(s) and equipment.