If I was starting it I would only be doing one session a week. I could definitely handle that. Grapling isn't that taxing, it's not like I'd be taking up rugby or boxing again.
Grappling is more taxing on your CNS and muscles than boxing is, for sure. Boxing you do a lot of cardio, but it's not that hard on your body. IMO, grappling can be really tough on your body.