I don't think it is anyone's right to have 24/7 access to my attention. It's not normal to be reacheable of every moment of every day. If its something more pressing sure, I understand that I really should message back at my earliest convenience. But if it's just every day chit chat and such then it can wait until I'm good and ready to open my mental space into "getting back" to everyone that keeps in touch on a regular basis. I genuinely have cultivated my friend group into people that understand that I enjoy my quiet and alone time. Instead of every other day to every day contact with my closest friends its weekly. And then face to face visiting monthly. People have busy lives and you are not the center of their world.

Edit: if it's a normal occurance with one or a couple of people in particular, just ask them how often they'd like to be "checked in" on. As stated above my friends and I are weekly check ins. But we've been known to go 3 weeks before getting nervous about safety. Depends on the person.