Some contracts do require time to get out of costume.
If an actor needs to get into costume you need to give them 30 minutes is a myth. This is not true, you are not required to give them a half-hour except for PERFORMANCES. (There are a lot good reasons why you might want to give them half-half). You can give them ten minutes to get dressed.
In tech situations, where you give half-hour, half-hour needs to be counted as work. (I know, it's odd) So often what I do during tech is give the actors 25 minutes to get dressed, give them a five minute break, and then we are set to do anything. (Some producers think that half-hour is not counted against break time . . . so, a 12:00n half-hour, they think the first break would be do at 1:50p. It is really do at 1:20p.)
The reason why some contracts have getting out of costume as part of work hours is, well, it's 12:00m end of the day, I should be able to just walk out. But, for some reason we count getting dressed as work, getting undressed doesn't count (on most contracts).
The keeping actors in costume is a tad bit disrespectful to your wardrobe crew - I tend to before any notes - to get them out of costume - usually saying, "You guys are on ten, and not required to be in costume at the end of the break . . . so if you want to get out of costume during your break - you are free to do so." It's amazing how often then we get out of costume.
But again, check your contract . . . as to if getting out of costume is required. If my cast asked for that, I would do an exchange, okay 15 minutes to get in costume, 15 minutes to get out of costume. And then I am not losing time.