I agree with Jenny that it is individual with each client, regarding how you choose to communicate with them. Most often I send emails out from time to time saying something along the lines of recently thinking of them and hoping they are well (and doing the homework I taught them - just to tease them :) Sometimes I even text my clients (and not just the younger ones!)
I picked up an interesting idea from a friend that I put on my intake forms "Do have permission to call or email you and leave messages with specific details regarding your appointments?" That may help establish the boundaries between you and your clients.