I've been on this Sub long enough to have grasped that the Romans didn't go around in togas literally all the time, but this makes me think, when would they have to wear a toga as opposed to just a tunic? If a Roman was going to a fancy dinner party, what would he wear? Or to put it more broadly, what was Rome's equivalent to the jeans-and-T-shirt, the business suit, and the tuxedo?
As for era, we'll call it Late Republic to Mid-Principate.
It sort of depends, but if you are an aristocrat in a city you will probably never be without your tunic and toga. It was vitally important to constantly communicate status, and so a Cicero would be no more likely to wear the equivalent of jeans and a t-shirt than George Washington. On the other hand, when Cicero visited his villa on the Bay of Naples he would wear Greek clothes, such as a chlamys (unfortunately I'm on my phone so can't link a picture, but you can Google it).
An ordinary Roman citizen would wear their toga to situations they need to communicate status, such as the forum or the theater. However, togas are not really suitable for manual labor so most of the time they would probably wear a tunic, basically a very long t-shirt. The point is that a toga marked one out as a citizen, so would be worn situations where that is important.