DM is a request for urgent attention, the social contract per team or per individual might be “respond to DMs immediately” but there’s no need to actually respond with the answer whatever question was asked.

As an example: my teams know that I check email twice a day (once on arriving, once in late afternoon), but I will respond to DMs about every half hour or so (Pomodoro time). There’s no promise of immediate response, because I have stuff I am actually doing.